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.

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

@@ -113,9 +113,9 @@ 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.
+**Review corrections (#1314 maintainer review) — two soundness items:**
+- **node-test fixture-existence guard (was fail-OPEN) — FIXED.** 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(path.resolve(cwd, fixture))` before spawning (symmetric fail-closed; resolved against the producer's `cwd` to match the child's resolution). **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` projection source (#1278 ↔ #1279 now COMPOSE) — DELIVERED.** Initially `descriptorFromProjection` reconstructed only `{ kind, target, rule? }` and the projection carried no fixture, so a prohibition wired purely through the deterministic path always hard-gated. This PR threads a **fourth flat scalar `check_violation_fixture`** through `projectProhibitions` + `descriptorFromProjection` (rides both kinds; mirrors `CheckDescriptor.violationFixture`). A prohibition authored with all four scalars now **machine-proves fail-first and greens end-to-end through the projection alone** (zero hand-authoring) — the round-trip is pinned by a fast-check property + CHK-03(D) + an end-to-end COMPOSE capstone. Fail-closed is preserved: a descriptor with no `check_violation_fixture` (or a blank one) projects absent and hard-gates. The remaining work under #1346 is now just the node-test causation residual above.
 
 ## Addendum (2026-06-15): optional `check` descriptor on the prohibition item — D3 shape extension (#1278)