fix(#1906): require node-test clean-fixture causation control (#2001)

* fix(#1906): require node-test clean-fixture causation control

The node-test fail-first proof accepted a deceptive content-independent
negative test — one that reds merely because GSD_PROHIB_SUBJECT is set,
ignoring the subject's content — whenever no cleanFixture was supplied,
because #1346's causation control was opt-in. The proof's observed signal
(RED) thus diverged from its target (RED caused by content) by default.

Make the causation control mandatory for the node-test kind: a descriptor
that omits cleanFixture is un-provable (fail-closed), never accepted under
the weaker violation-only proof. When a clean fixture is present, fail-first
is proven exactly as before (RED on violation AND non-vacuous GREEN on clean).
The lint-rule kind is unchanged (its subject IS the linted file; no
GSD_PROHIB_SUBJECT indirection).

Breaking (Hyrum): a previously-green node-test prohibition with no clean
fixture now hard-gates — blast radius is zero in-tree (no node-test
prohibition ships today; only the lint-rule local/no-source-grep dogfood).

Supersedes ADR-1606 Decision 4 / ADR-550 #1346 addendum's opt-in.

Closes #1906

Claude-Session: https://claude.ai/code/session_017vYn26e3nkDNxcpty1ciPJ

* docs(#1906): supersede the #1346 opt-in causation control (mandatory for node-test)

Record the node-test mandatory-causation-control supersede across the
governing surfaces:

- ADR-1606 (the enforcement decision-of-record): addendum + Decision 4
  annotated + the "Mandatory causation control — REJECTED" alternative
  flipped to accepted (premise no longer holds: zero in-tree node-test
  consumers).
- ADR-550: the 2026-06-21 #1346 "Why opt-in, not required" paragraph
  marked SUPERSEDED, pointing at ADR-1606.
- spec-phase.md: check_clean_fixture is now REQUIRED for node-test
  (was "optional").
- CONTEXT.md: PROHIB.enforce.causation predicate updated.

Regenerated the shipped-artifact cascade from the spec-phase.md edit
(+149 B, well under the 40960 cap): 16 golden-install-parity fixtures
and the workflow size baseline.

Refs #1906

Claude-Session: https://claude.ai/code/session_017vYn26e3nkDNxcpty1ciPJ

CONTEXT.md

@@ -387,7 +387,7 @@ The prompt-level data/instruction isolation seam for untrusted web/document ingr
 `PROHIB.enforce.green-rule=passed iff provenFailFirst===true && run.passed===true (runProhibitionEnforcement); every miss/fail/un-provable HARD-GATES both modes via dispositionForProhibition's fail-closed default`
 `PROHIB.enforce.kinds=node-test (non-vacuous red via isNonVacuousNodeTestRed; pass-side vacuity via isNonVacuousNodeTestPass) | lint-rule (eslint --format json filtered by ruleId)`
 `PROHIB.enforce.failfirst=MACHINE-PROVEN against an author-supplied violation fixture (#1279); caller failFirst attestation DEMOTED to a non-authoritative hint (FF-08)`
-`PROHIB.enforce.causation=opt-in clean-fixture control proves the red is content-caused not env-var-set (#1346); absent=documented residual`
+`PROHIB.enforce.causation=clean-fixture control proves the red is content-caused not env-var-set; MANDATORY for node-test (#1906 supersedes #1346 opt-in) — absent clean-fixture ⇒ node-test un-provable/fail-closed; lint-rule needs none (its subject IS the linted file)`
 `PROHIB.descriptor.shape=5 FLAT scalars (check_kind,check_target,check_rule,check_violation_fixture,check_clean_fixture) — NEVER a nested check:{} (parseMustHavesBlock is a flat parser, src/frontmatter.cts)`
 `PROHIB.rail=core verify rail, non-toggleable (ADR-857 verification-substrate boundary / decision #6); the verifier<->predicate contract is NOT an off-by-default capability`
 `PROHIB.judgment-tier=never-silent / never-hard-halt soft gate; autonomous emits "unverified-prohibition — human review recommended" (exogenous grading, ADR-550 D4)`