enhance(verify-phase): node-test causation control — prove the RED is content-caused (#1346)

The #1279 node-test machine-proof confirmed a known-bad subject drives the
negative test RED, but could not distinguish a genuine content-violation from a
deceptive test that reds merely because GSD_PROHIB_SUBJECT is set. Add an
optional fifth flat scalar `check_clean_fixture` (-> CheckDescriptor.cleanFixture)
threading a KNOWN-CLEAN control subject through projectProhibitions +
descriptorFromProjection. When present, the prover also runs the check against
the clean subject and requires GREEN, so fail-first is proven only when the check
is RED on the violation AND GREEN on the clean subject (content-dependent).

Opt-in and additive: absent a clean fixture the prover behaves exactly as
post-#1314 (no control, documented residual), preserving the zero-authoring
compose path; the lint-rule kind needs no analog (its subject IS the linted
file, no env indirection). Coverage: RED-first deceptive case, positive,
missing-clean fail-closed, round-trip read-back/emit, fast-check property
extended to the 5th scalar, and an end-to-end COMPOSE capstone (honest vs
deceptive). Docs: ADR-550 dated addendum, prohibition-probe reference,
spec-phase + verify-phase workflows.

Closes #1346

Claude-Session: https://claude.ai/code/session_01GsPRb8zvpcT7Eat6vZw8PX

tests/workflow-size-baseline.json

@@ -72,7 +72,7 @@
   "ship.md": 24388,
   "sketch-wrap-up.md": 14223,
   "sketch.md": 19960,
-  "spec-phase.md": 30921,
+  "spec-phase.md": 31503,
   "spike-wrap-up.md": 15092,
   "spike.md": 24517,
   "stats.md": 6718,
@@ -85,6 +85,6 @@
   "undo.md": 10431,
   "update.md": 21053,
   "validate-phase.md": 10745,
-  "verify-phase.md": 37821,
+  "verify-phase.md": 38228,
   "verify-work.md": 31157
 }