enhance(#1279): project check_violation_fixture scalar — #1278 locate + #1279 proof compose end-to-end (#1346)

Delivers option (a) from the #1314 maintainer review: thread a fourth flat
scalar check_violation_fixture through the projection so a prohibition authored
at spec-phase machine-proves fail-first and greens through the deterministic
path alone — zero hand-authoring at verify time.

- src/probe-core.cts: Prohibition gains check_violation_fixture?; projectProhibitions
  emits it (both kinds) ONLY for a well-formed descriptor and ONLY when non-empty
  (blank/absent -> projects absent -> producer hard-gates, never a partial green).
- src/prohibition-enforcement.cts: descriptorFromProjection reads it back into
  violationFixture via the same numeric-coercion-safe scalar() normalizer.
- Tests (RED-first, proven non-vacuous by reverting both src edits): CHK-02(#1346)
  projection emit, CHK-08(#1346) read-back, CHK-03(D) example round-trip, the
  fast-check round-trip property extended to the 4th scalar (the contract trek-e
  blocked #1301 on), and a real-subprocess COMPOSE capstone greening end-to-end
  through project -> descriptorFromProjection -> default prover+runner.
- Docs flipped from 'hard-gates until #1346' to 'composes end-to-end': verify-phase.md,
  prohibition-probe.md, spec-phase.md authoring, ADR-550 addendum, changeset.
  #1346 now tracks only the node-test causation residual.

190 affected-suite tests green; eslint + tsc clean; size baseline regenerated.

.changeset/1279-machine-proven-fail-first.md

@@ -5,6 +5,6 @@ pr: 1314
 
 **Test-tier prohibition fail-first is now MACHINE-PROVEN, not caller-attested** — the deferred literal `regression-must-fail-first` property of ADR-550 Decision 4 (the gap #1259 / PR #1273 left as a tracked follow-up) has landed. The `check prohibition-enforcement` producer (`src/prohibition-enforcement.cts`, compiled by `build:lib` to the gitignored `gsd-core/bin/lib/prohibition-enforcement.cjs`) no longer trusts the caller's `failFirst` attestation: before a clean, non-vacuous pass can dispose a `test`-tier prohibition green, the new `defaultProveFailFirst` prover independently RUNS the wired check against a KNOWN VIOLATION and confirms it goes RED. Attestation is gone from the green AND (`passed = proof.provenFailFirst === true && run.passed === true`); any other outcome — passes-on-violation, can't-prove, throws, times out, or no violation source — **hard-gates in BOTH interactive and autonomous modes** (ADR-550 D4 / D3). The evidence record gains a `failFirstProof` field recording HOW fail-first was proven (FF-07). A caller can no longer green a toothless check.
 
-The violation is sourced from a new descriptor field, **`CheckDescriptor.violationFixture`** — an author-supplied path to a known-bad subject. For a `lint-rule` the prover lints that fixture and requires the rule id to appear in the JSON report (the rule must have teeth); for a `node-test` the prover spawns the negative test with the subject injected through the **`GSD_PROHIB_SUBJECT`** env convention and requires a NON-VACUOUS red — `# fail >= 1` AND a failing test named distinctly from the file (`isNonVacuousNodeTestRed`), so a violation fixture that merely CRASHES the test at load is not mistaken for the negative assertion firing red (symmetric with the clean-pass non-vacuity guard). The node-test prover also requires the `violationFixture` to EXIST (resolved against `cwd`) before spawning — a missing/typo'd path fail-CLOSES rather than letting an honest test's ENOENT crash forge a green (symmetric with the lint path's file-result guard). A documented residual — proving the red was *caused by* the violation rather than by the env merely being set — and threading a `check_violation_fixture` scalar through the #1278 projection so the deterministic path can green end-to-end are both tracked as follow-up **#1346**. The `lint-rule` path is fully shippable now and is dogfooded against the in-tree `local/no-source-grep` rule; the `node-test` path ships its mechanism (a fixture-bearing descriptor IS machine-proven) and is exercised by SYNTHETIC temp fixtures — there is no live in-tree `node --test` prohibition to dogfood. `CheckDescriptor.failFirst` is **DEMOTED, not removed** (FF-08): it is kept as a non-authoritative hint so the #1259 route-JSON shape and the `CheckDescriptor` type stay backward-compatible mid-migration, but no path greens on it alone. The green/fail-closed policy in `src/probe-core.cts` (`dispositionForProhibition`, reads only `evidence.length > 0`) is untouched; the evidence array shape is additive. This closes ADR-550's D5d follow-up — see the dated 2026-06-15 ADR-550 addendum. (#1279)
+The violation is sourced from a new descriptor field, **`CheckDescriptor.violationFixture`** — an author-supplied path to a known-bad subject. For a `lint-rule` the prover lints that fixture and requires the rule id to appear in the JSON report (the rule must have teeth); for a `node-test` the prover spawns the negative test with the subject injected through the **`GSD_PROHIB_SUBJECT`** env convention and requires a NON-VACUOUS red — `# fail >= 1` AND a failing test named distinctly from the file (`isNonVacuousNodeTestRed`), so a violation fixture that merely CRASHES the test at load is not mistaken for the negative assertion firing red (symmetric with the clean-pass non-vacuity guard). The node-test prover also requires the `violationFixture` to EXIST (resolved against `cwd`) before spawning — a missing/typo'd path fail-CLOSES rather than letting an honest test's ENOENT crash forge a green (symmetric with the lint path's file-result guard). The deterministic spec→verify path composes end-to-end: a fourth flat scalar **`check_violation_fixture`** is projected by `projectProhibitions` and read back by `descriptorFromProjection` (rides both kinds), so a prohibition authored with all four `check_*` scalars machine-proves fail-first and greens through the projection alone — zero hand-authoring at verify time (#1278 + #1279 + #1346; round-trip pinned by a fast-check property + CHK-03(D) + an end-to-end COMPOSE capstone). One documented residual remains under **#1346**: the node-test proof confirms the fixture exists and the check reds, but cannot generically prove the red was *caused by* the subject's content rather than by the env merely being set. The `lint-rule` path is fully shippable now and is dogfooded against the in-tree `local/no-source-grep` rule; the `node-test` path ships its mechanism (a fixture-bearing descriptor IS machine-proven) and is exercised by SYNTHETIC temp fixtures — there is no live in-tree `node --test` prohibition to dogfood. `CheckDescriptor.failFirst` is **DEMOTED, not removed** (FF-08): it is kept as a non-authoritative hint so the #1259 route-JSON shape and the `CheckDescriptor` type stay backward-compatible mid-migration, but no path greens on it alone. The green/fail-closed policy in `src/probe-core.cts` (`dispositionForProhibition`, reads only `evidence.length > 0`) is untouched; the evidence array shape is additive. This closes ADR-550's D5d follow-up — see the dated 2026-06-15 ADR-550 addendum. (#1279)
 
 **PR-review flag — `GSD_PROHIB_SUBJECT` + `violationFixture` are PROPOSED, renamable conventions.** Both are net-new surface with ZERO live in-tree consumers (no in-tree node-test prohibition yet; node-test proof runs only on synthetic test fixtures, the real dogfood stays the lint-rule). They are forward-looking scaffolding, so a later rename — or replacing the env var with an argv — is a mechanical, zero-migration find/replace. Surfacing them here so the maintainer can **ratify, rename, or replace them at PR review** with no migration cost, exactly as #1278's ADR addendum was reviewed at PR time. The `failFirst` demotion is likewise open to weighing outright removal; the keep-as-hint rationale is recorded in the ADR addendum.