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  marked SUPERSEDED, pointing at ADR-1606.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

gsd-core/workflows/spec-phase.md

@@ -371,11 +371,13 @@ For each Requirement gathered so far, run the two-stage recall→precision pass:
          against to **machine-prove fail-first**; rides BOTH kinds. Capture it to let the item green
          end-to-end with zero hand-authoring at verify time; for `node-test` the negative test should
          read its subject from the `GSD_PROHIB_SUBJECT` env var so the prover can inject this fixture.
-       - `check_clean_fixture` (#1346) — **optional** path to a KNOWN-CLEAN control subject. When
-         captured, the `node-test` prover also runs the check against it and requires GREEN — proving
-         the violation's RED is caused by the subject's *content*, not by `GSD_PROHIB_SUBJECT` merely
-         being set. Capture it for a stronger guarantee; omit it and the check still proves fail-first
-         on the violation alone (the content-causation residual stays documented for that case).
+       - `check_clean_fixture` (#1346; **REQUIRED for `node-test` as of #1906**) — path to a
+         KNOWN-CLEAN control subject. The `node-test` prover runs the check against it and requires
+         GREEN — proving the violation's RED is caused by the subject's *content*, not by
+         `GSD_PROHIB_SUBJECT` merely being set. For a `node-test` this is **mandatory**: omit it and
+         the check is un-provable (fail-closed), never proven on the violation alone — so a deceptive
+         content-independent test cannot pass. (`lint-rule` needs no clean fixture: its subject IS the
+         linted file, no `GSD_PROHIB_SUBJECT` indirection.)
        This is a **SOFT capture (CHK-04): a `test`-tier prohibition WITHOUT a descriptor is still
        allowed** — if the author cannot yet name the wired check, leave the descriptor empty and
        proceed. It is NOT a hard authoring block; the item simply stays fail-closed/flagged