fix(#1279): node-test prover fail-closes on missing violationFixture + honest projection docs

Addresses the #1314 maintainer review (trek-e):

- Major 1 (fail-OPEN): defaultProveFailFirst's node-test branch only guarded
  `if (!fixture)`. A missing/typo'd/stale violationFixture made GSD_PROHIB_SUBJECT
  point at a missing file; an honest negative test threw ENOENT *inside its
  callback* (a failing test named distinctly from the file), which
  isNonVacuousNodeTestRed accepted as proof -> a green forged from a setup crash.
  Now requires fs.existsSync(path.resolve(cwd, fixture)) before spawning, symmetric
  with the lint-rule path's file-result guard. Regression test pins it (RED without
  the guard); a second test pins cwd-relative fixture resolution.

- Major 2 (misleading prose): the #1278 projection carries no violationFixture, so
  the deterministic-locate path always hard-gates (fail-closed) until a
  check_violation_fixture scalar is threaded through. verify-phase.md and
  prohibition-probe.md no longer read as if the projected path produces greens;
  the ADR-550 addendum records both items. Tracked as follow-up #1346.

- Documented residual: existence is necessary but not sufficient (a red caused by
  the env being set vs the subject's content); recorded as a constraint, in #1346.

- Nit: stale 'NOT attested fail-first' comment -> 'NOT machine-proven fail-first'.

64 tests pass; eslint + tsc clean; changeset valid.

.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 `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). 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)
 
 **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.