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.

docs/adr/550-spec-phase-probe-contract.md

@@ -113,6 +113,10 @@ This addendum ratifies three contract points:
 
 Net effect on D4: the *guarantee* ("a `test`-tier prohibition is never a silent pass") was preserved at every step — fail-closed-now (#644), genuine-execution (#1259), and now **machine-proven fail-first (#1279)**. A `test`-tier prohibition reaches `green`/`passed` ONLY when the wired check both genuinely, non-vacuously passes AND is independently proven to fail on a violation; every miss/fail/un-provable hard-gates. The decision also lives in `src/prohibition-enforcement.cts` comments, `gsd-core/references/prohibition-probe.md`, `gsd-core/workflows/verify-phase.md`, and the #1279 changeset.
 
+**Review corrections (#1314 maintainer review) — two soundness items, tracked:**
+- **node-test fixture-existence guard (was fail-OPEN).** The node-test prover originally guarded only `if (!fixture)`. A missing/typo'd/stale `violationFixture` path made `GSD_PROHIB_SUBJECT` point at a non-existent file; an honest negative test then threw ENOENT *inside its callback* — a failing test named distinctly from the file — which `isNonVacuousNodeTestRed` accepted as proof, **forging a green from a setup crash** (asymmetric with the lint-rule path, which fail-CLOSES on `< 1` file result). Fixed by requiring `fs.existsSync(fixture)` before spawning (symmetric fail-closed). **Documented residual (#1346):** existence is necessary but not sufficient — a deceptive test that reds merely *because* `GSD_PROHIB_SUBJECT` is set (not because the subject's CONTENT violates) is still accepted; proving causation generically for an arbitrary author-supplied test is not possible, so it is recorded as a constraint, not implied-solved.
+- **`violationFixture` has no projection source (#1278 ↔ #1279 do not yet compose).** `descriptorFromProjection` reconstructs only `{ kind, target, rule? }`; the projected `check_*` scalars carry **no** `violationFixture`. Since green now *requires* a fixture, a prohibition wired purely through the #1278 deterministic path **always hard-gates (fail-closed, safe)** until a `check_violation_fixture` scalar is threaded through — tracked as **#1346**. Until then green requires a hand-supplied `violationFixture`; `verify-phase.md` now states this explicitly rather than implying the projected path produces greens.
+
 ## Addendum (2026-06-15): optional `check` descriptor on the prohibition item — D3 shape extension (#1278)
 
 This ratifies the **deterministic SOURCE** for the test-tier `CheckDescriptor` that #1259 (PR #1273) left caller/verifier-supplied. #1259 shipped the PRODUCER (`check prohibition-enforcement`) that *runs* a wired check given a `{kind, target, rule?}` descriptor, but the descriptor itself was invented by the verify-phase LLM each run (the "locate" half). #1278 makes that locate half **deterministic**: an optional `check` descriptor is authored at spec-phase on the resolved `test`-tier prohibition, projected by `projectProhibitions`, and read back by verify-phase — so a wired, passing test closes the gap with **zero manual authoring**. This extends the **Decision 3 prohibition-item shape** (it adds optional keys to that item), so it is ratified here rather than rewriting D3 in place.