feat(#1154): honest verifier — abstain (insufficient_spec) on non-inferable backstop truths (#1738)

* feat(verify-phase): honest verifier — abstain (insufficient_spec) on non-inferable backstop truths (#1154)

Carry the edge-probe's existing `backstop` (non-inferable) tier through the
plan-phase projection as a structured flat-scalar marker instead of a prose
parenthetical, and make verify-phase abstain -> human_needed (never silent-pass)
on a backstop truth it cannot confirm with explicit evidence. Truth-axis mirror
of #644's prohibition judgment-tier (ADR-550 D4).

Engine (deterministic, CI-tested per ADR-550 D5 — never the LLM verdict):
- src/probe-core.cts: truthStatement/truthVerification normalizers, projectTruths
  (conservative serializer), dispositionForUnverifiableTruth (backstop+no-evidence
  -> unverified/flagged/insufficient_spec; backstop+evidence -> green; inferable
  -> green, the over-abstention guard).
- src/roadmap.cts: coerceTruthToString now reads `statement` first so an object-form
  backstop truth is surfaced, not dropped (Hyrum backward-compat for truth-readers).

Workflow/agent/docs: plan-phase emits the structured marker (flat scalar, ADR-550
#1278); verify-phase + gsd-verifier add the abstain arm; new references/honest-verifier.md;
FEATURES/COMMANDS document insufficient_spec; ADR-550 amended (truth-axis D4 mirror).

Decisions adopted (trek-e review): insufficient_spec feeds existing human_needed with a
distinguishable reason (no new VERIFIER_STATUS); changeset Changed; round-trip parity
test; abstain-on-unconfirmed-backstop regression test red-first.

Implementation notes (deviations from the issue's proposed file list, verified live):
- frontmatter.cts needs no change — its flat parser already round-trips object-form truths.
- verify.cts needs no change — it grades artifacts/key_links structurally; truths are
  LLM-graded at the workflow layer, so consumption lives there + the deterministic helper.
- No CJS<->SDK hand-sync — the SDK seam was retired (ADR-0174); src/*.cts is sole source.

Regenerated artifacts: golden-install-parity fixtures, INVENTORY-MANIFEST, size baselines.

* chore(#1154): add changeset (Changed) for honest verifier

User-facing changelog fragment for #1738. Typed `Changed` (not `Added`) per
trek-e review condition 3 — the verify behavior shifts for backstop-bearing specs
(a confident silent `passed` becomes `human_needed`), which is user-visible even
though the schema marker is additive.

* docs(#1154): score-formula also excludes abstained insufficient_spec truths (review nit-1)

trek-e review nit: the verify-phase score sentence said PRESENT_BEHAVIOR_UNVERIFIED
truths were "the only ones excluded" from verified_truths. Post-#1154 an abstained
`insufficient_spec` backstop truth is also excluded (it is not ✓ VERIFIED and routes
to human_needed). Behavior was already correct; this tightens the wording.
Regenerated golden-install-parity fixtures + workflow-size baseline for the touched
verify-phase.md. (Nit-2 — a dedicated insufficient_spec_items frontmatter list — is
intentionally not taken: the current design is ADR-550-D4-conformant, the abstain
cause rides as a distinguishable report reason, and adding it would exceed the
approved scope.)

---------

Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

gsd-core/workflows/plan-phase.md

@@ -912,7 +912,7 @@ Output consumed by /gsd:execute-phase. Plans need:
 - Tasks in XML format with read_first and acceptance_criteria fields (MANDATORY on every task)
 - Verification criteria
 - must_haves for goal-backward verification
-- If the SPEC has an `## Edge Coverage` section, lift every `covered` edge's acceptance criterion into `must_haves.truths`, and every `backstop` edge into `must_haves.truths` as a non-inferable check (note it needs a held-out/property-based test). `unresolved` edges are explicit assumptions — surface them in the plan, do not silently drop them.
+- If the SPEC has an `## Edge Coverage` section, lift every `covered` edge's acceptance criterion into `must_haves.truths` as a plain string, and every `backstop` edge **as a structured flat-scalar marker** — an object item `{ statement: <the check>, verification: backstop }`, NOT a prose note (the verifier branches deterministically on the `verification: backstop` field; a parenthetical is unparseable — the #1110 fragility). Use a flat scalar `verification:` continuation key, never a nested object (ADR-550 #1278). At verify time a `backstop` truth the verifier cannot confirm with explicit evidence abstains → `human_needed` (reason `insufficient_spec`), never a silent pass (#1154; see `references/honest-verifier.md`). `unresolved` edges are explicit assumptions — surface them in the plan, do not silently drop them.
 - If the SPEC has a `## Prohibitions` section, lift every resolved prohibition into the `must_haves.prohibitions:` sibling block (NOT `truths` — ADR-550 D3) carrying `statement` + `status` + `verification`; unresolved prohibitions are explicit assumptions — surface them in the plan, do not silently drop them. A prohibition is a must-NOT (negative) check that belongs in its own `must_haves.prohibitions` block. Never place a must-NOT under `must_haves.truths` — that block keeps positive-observable semantics only.
 - **"Artifacts this phase produces" section (MANDATORY)** — list every symbol this phase creates: decorators, classes, functions, CLI flags, struct/dataclass fields, new file paths. The plan-review-convergence source-grounding pass reads this section to exclude newly-created symbols from drift verification; omitting it causes new symbols to be flagged for acknowledgement.
 </downstream_consumer>

gsd-core/workflows/verify-phase.md

@@ -117,6 +117,8 @@ For each truth: identify supporting artifacts → check artifact status → chec
 
 **Behavior-dependent truths:** when a truth asserts a state transition or a cancellation/cleanup/ordering invariant, symbol presence + wiring is necessary but not sufficient — the code can be present and wired yet still leak state on the path the invariant covers. Mark such a truth ✓ VERIFIED only when a pre-existing test exercises the transition/invariant and passes (one named test, never the full suite); otherwise mark it ⚠️ PRESENT_BEHAVIOR_UNVERIFIED, emit a human-verification item, and exclude it from the verified score.
 
+**Non-inferable (`backstop`) truths (#1154):** a `must_haves.truths` item in object form `{ statement, verification: backstop }` is non-inferable — the correct behavior is not derivable from the spec alone, so the verifier cannot self-detect the gap and would false-pass it confidently. Branch on the `verification: backstop` field (read via `truthVerification()`, never prose): if confirmable with **explicit evidence** (a passing wired held-out/property test, or a directly-observed behavior) → ✓ VERIFIED; otherwise **abstain** — mark ⚠️ `insufficient_spec`, emit an `unverified — held-out test recommended` human-verification item, exclude from the verified score (routes to `human_needed`). Exogenous only (never a self-judged "abstain if unsure"); an inferable truth is never abstained. See `references/honest-verifier.md`.
+
 **Example:** Truth "User can see existing messages" depends on Chat.tsx (renders), /api/chat GET (provides), Message model (schema). If Chat.tsx is a stub or API returns hardcoded [] → FAILED. If all exist, are substantive, and connected → VERIFIED.
 </step>
 
@@ -488,17 +490,22 @@ Classify status using this decision tree IN ORDER (most restrictive first):
    - **judgment-tier, autonomous run** (non-authoritative LLM-judge verdict): emit the `unverified-prohibition — human review recommended` flag and classify → **human_needed** (autonomous completion reads "complete with N flagged prohibitions"; never a silent pass, never a hard halt).
    - **judgment-tier, interactive run**: route to the end-of-phase human checkpoint → **human_needed**.
 
-3. IF the previous step produced ANY human verification items — this includes every ⚠️ PRESENT_BEHAVIOR_UNVERIFIED truth:
+2b. IF any `must_haves.truths` item carries the `verification: backstop` marker (#1154 — the verify-time truth-axis mirror of ADR-550 D4) AND the verifier cannot confirm it with **explicit evidence** (a wired held-out/property-based test that PASSES, or a directly-observed behavior — i.e. `dispositionForUnverifiableTruth()` returns `status: 'unverified'`, `flagged: true`, `reason: 'insufficient_spec'`):
+   - **abstain → human_needed**, NEVER `passed` and never silently graded green. Emit a prominent `unverified — held-out test recommended` flag carrying the distinguishable `reason: insufficient_spec` (so it is not conflated with ordinary manual-UAT `human_needed`).
+   - *Autonomous run:* record it and continue — completion reads "complete with N unverified non-inferable checks"; never a hard halt of an AFK run. *Interactive run:* route to the end-of-phase human checkpoint.
+   - **Exogenous only:** abstention fires SOLELY on the `backstop` tag, never a self-judged "abstain if unsure" (N17). An **inferable** truth is NEVER abstained (over-abstention guard); a `backstop` truth WITH a passing wired held-out test reaches **passed**. Reliable on capable tiers (`sonnet`+); the budget `haiku` tier degrades — see `references/honest-verifier.md`.
+
+3. IF the previous step produced ANY human verification items — this includes every ⚠️ PRESENT_BEHAVIOR_UNVERIFIED truth and every abstained `insufficient_spec` backstop truth:
    → **human_needed** (even if all other truths VERIFIED)
 
-4. IF all checks pass AND no human verification items AND no flagged prohibitions:
+4. IF all checks pass AND no human verification items AND no flagged prohibitions AND no abstained (`insufficient_spec`) truths:
    → **passed**
 
-**passed is ONLY valid when no human verification items AND no flagged prohibitions exist.** A prohibition (must-NOT) can never be silently absorbed into a `passed` verdict — that is the core failure mode ADR-550 D4 forbids.
+**passed is ONLY valid when no human verification items, no flagged prohibitions, AND no abstained `insufficient_spec` truths exist.** Neither a prohibition (must-NOT) nor an unconfirmable non-inferable truth can ever be silently absorbed into a `passed` verdict — that is the core failure mode ADR-550 D4 forbids (now closed on both the prohibition and truth axes).
 
 A ⚠️ PRESENT_BEHAVIOR_UNVERIFIED truth is never FAILED and never VERIFIED: it does not trigger gaps_found (the code is present and wired) and is not counted as verified (its runtime behavior was not exercised). It routes through the existing human_needed sink — no new overall status.
 
-**Score:** `verified_truths / total_truths` — `verified_truths` counts ✓ VERIFIED truths plus PASSED (override) truths; ⚠️ PRESENT_BEHAVIOR_UNVERIFIED truths are the only ones excluded, reported separately as the `behavior_unverified` count. A headline N/N therefore certifies behavioral evidence for every behavior-dependent truth, not merely symbol presence.
+**Score:** `verified_truths / total_truths` — `verified_truths` counts ✓ VERIFIED truths plus PASSED (override) truths; excluded are ⚠️ PRESENT_BEHAVIOR_UNVERIFIED truths (the `behavior_unverified` count) and abstained ⚠️ `insufficient_spec` backstop truths (#1154) — both are not ✓ VERIFIED and both route to `human_needed`. A headline N/N therefore certifies behavioral evidence for every behavior-dependent truth and explicit evidence for every non-inferable one, not merely symbol presence.
 </step>
 
 <step name="filter_deferred_items">