test(U9): add a core unit-gate so dependency gating and FN-5819 block merges (#2569)

**U9, PR10.** Two `package.json` lines. No test or production changes —
this only decides *when* existing tests run.

## The gap

Two U9 safeguards are well covered but sat in **no blocking gate**.
Their proof lives in `packages/core/src/__tests__/task-merge.test.ts`,
and core's only gate job is `test:pg-gate` (two PG tests). A regression
in either surfaced in non-blocking full-suite — after the merge.

## What's now gated, each verified by mutation delta

**`task-merge.test.ts`**

| Invariant | Mutation | NEW failures |
|---|---|---|
| Safeguard 3 — dependency gating | `getTaskCompletionBlocker` drops the
unresolved-dependency reason | **5** |
| FN-5819 — exception bounded to a live group | drop `group.status ===
"open"` | **1** |
| FN-5819 — exception bounded to shared members | widen
`isSharedBranchGroupMemberIntegration` to every task | **4** |

Both FN-5819 directions matter. This is the **only** scoped exception to
`autoMerge:false`, so its *narrowness* is the invariant — not merely its
existence. A test that only proves the exception works would pass while
the exception swallowed every task.

**`legacy-adoption.test.ts`**

| Invariant | Mutation | NEW failures |
|---|---|---|
| FN-8492 — orphaned pending results REWRITTEN to failed, never DELETED
| delete instead of rewrite | **2** |

That one matters because deletion *silently satisfies* the merge gate:
the gate blocks on pending/failed results, not on an enabled step with
no result, so deleting lets a task merge with its review skipped.

## Implementation

Adds `packages/core` → `test:unit-gate`, a curated **non-PG allow-list**
mirroring `engine-core`'s discipline (explicit membership, not a glob),
run as a third parallel job in the root `test:gate` block alongside the
engine and PG jobs.

**Gate fires — verified, not assumed:**
- drop the dependency reason → `pnpm test:gate` **exits 1**
- drop the FN-5819 open-group bound → **exits 1**
- restored → **exits 0**

## Cost: no measurable increase

| | Runs |
|---|---|
| baseline | 13.07s, 14.95s |
| with the job | 12.20s, 12.58s |

It runs in parallel with the existing jobs and finishes well inside
them, so the delta sits inside run-to-run variance. **I am not claiming
a speedup** — the honest reading is "no measurable cost", and the
variance band here is wider than the change.

## Reversible call made rather than asked

A new `test:unit-gate` script rather than widening `test:pg-gate` or
adding a glob. `test:pg-gate` carries PG setup these pure unit tests do
not need, and a glob would admit all of core by default — which
AGENTS.md explicitly forbids ("tests never graduate into the gate by
default"). Membership stays explicit so the next addition has to state
its evidence.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -21,7 +21,7 @@
     "check:changesets": "node scripts/check-changeset-format.mjs",
     "check:quarantine-ledger": "node scripts/check-quarantine-ledger.mjs",
     "check:mock-completeness": "node scripts/check-mock-completeness.mjs",
-    "test:gate": "node scripts/check-no-nohup.mjs && node scripts/check-no-cwd-relative-dashboard-test-reads.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-getdatabase.mjs && node scripts/check-capacity-pool-id.mjs && node scripts/check-no-node-only-core-imports-in-dashboard.mjs && node scripts/check-pi-versions-pinned.mjs && node scripts/check-no-test-timeout-appeasement.mjs && node scripts/check-changeset-format.mjs && node scripts/check-mock-completeness.mjs && sh -c 'pnpm --filter @fusion/engine test:core & engine_pid=$!; pnpm --filter @fusion/core test:pg-gate & pg_pid=$!; status=0; wait $engine_pid || status=1; wait $pg_pid || status=1; exit $status' && pnpm --filter @runfusion/fusion test:ci-shape",
+    "test:gate": "node scripts/check-no-nohup.mjs && node scripts/check-no-cwd-relative-dashboard-test-reads.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-getdatabase.mjs && node scripts/check-capacity-pool-id.mjs && node scripts/check-no-node-only-core-imports-in-dashboard.mjs && node scripts/check-pi-versions-pinned.mjs && node scripts/check-no-test-timeout-appeasement.mjs && node scripts/check-changeset-format.mjs && node scripts/check-mock-completeness.mjs && sh -c 'pnpm --filter @fusion/engine test:core & engine_pid=$!; pnpm --filter @fusion/core test:pg-gate & pg_pid=$!; pnpm --filter @fusion/core test:unit-gate & unit_pid=$!; status=0; wait $engine_pid || status=1; wait $pg_pid || status=1; wait $unit_pid || status=1; exit $status' && pnpm --filter @runfusion/fusion test:ci-shape",
     "smoke:boot": "node scripts/boot-smoke.mjs",
     "local": "node scripts/start-local.mjs",
     "dev": "node scripts/dev-with-memory.mjs",

packages/core/package.json

@@ -48,7 +48,8 @@
     "typecheck": "tsc --noEmit",
     "test": "vitest run --silent=passed-only --reporter=dot",
     "test:embedded-postgres": "vitest run src/__tests__/postgres/embedded-lifecycle.test.ts --silent=passed-only --reporter=dot",
-    "test:pg-gate": "vitest run --config vitest.pg.config.ts src/__tests__/postgres/handoff-to-review-atomicity.pg.test.ts src/__tests__/postgres/task-lifecycle-e2e.pg.test.ts --silent=passed-only --reporter=dot"
+    "test:pg-gate": "vitest run --config vitest.pg.config.ts src/__tests__/postgres/handoff-to-review-atomicity.pg.test.ts src/__tests__/postgres/task-lifecycle-e2e.pg.test.ts --silent=passed-only --reporter=dot",
+    "test:unit-gate": "vitest run src/__tests__/task-merge.test.ts src/__tests__/legacy-adoption.test.ts --silent=passed-only --reporter=dot"
   },
   "devDependencies": {
     "@earendil-works/pi-coding-agent": "0.82.1",