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>

docs/INVENTORY.md

@@ -308,6 +308,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `domain-probes.md` | Domain-specific probing questions for discuss-phase. |
 | `edge-probe.md` | Spec-phase edge-completeness probe — 8-category edge taxonomy, shape classification, and the `requirements → checks → verifier` resolution model (Step 5.5). |
 | `prohibition-probe.md` | Spec-phase prohibition-completeness probe — the two-stage adversarial-recall → precision protocol that surfaces the unwritten *must-NOT* constraints (values/safety/ethics), with status×verification (`test`/`judgment`) tiering and canon-referral breadcrumbs (Step 5.6); second adapter of the `probe-core` resolution model. |
+| `honest-verifier.md` | Verify-time abstention on non-inferable (`backstop`) truths — the truth-axis mirror of the prohibition judgment-tier disposition (ADR-550 D4): a `backstop` truth the verifier can't confirm with explicit evidence abstains → `human_needed` (reason `insufficient_spec`), never a silent pass (#1154). |
 | `gate-prompts.md` | Gate/checkpoint prompt templates. |
 | `loop-hook-dispatch.md` | Generic dispatch contract for consuming `gsd_run loop render-hooks <point> --raw` output in any host-loop workflow — envelope shape, per-kind dispatch rules (contribution/step/gate), and liveness banner. |
 | `scout-codebase.md` | Phase-type→codebase-map selection table for discuss-phase scout step (extracted via the discuss-phase/modes progressive-disclosure split, #717). |