gate: run the inert-sync-lane ratchet in test:gate — it was wired to nothing (#3136)

## A gate that nothing runs

`check:inert-sync-lanes` has existed since #3062 and been hardened three
times (#3068, #3079, and #3122 pending). It is invoked by **nothing** —
not `test:gate`, not any workflow. It is a `package.json` script that
only runs if a human types it.

Here is the round trip that cost, on `main`:

| | |
|---|---|
| **#3108** | flagged `triage.ts:765`: *"the obvious next move is to
convert the third with the same helper. **That would be wrong twice
over.**"* Both blockers named, backed by
`sync-workflow-ir-second-blocker.test.ts`. |
| **#3114** | converted exactly that arm, replaced the warning with its
own note, reported census 45 → 44. |
| **#3126** | reverted it — after someone ran the ratchet by hand and
found `main` red. |

Hours apart. All three green in CI. **No behaviour changed on any board
at any point.** The ratchet flagged it correctly the entire time;
nothing ran it, so a caught defect cost three PRs instead of one failed
check.

This is not a criticism of #3114's author. A written warning at the
exact line, with a test behind it, was overwritten within hours by a
well-intentioned change — that is simply what unenforced prose does
under fleet pressure. It is the fourth time this class has landed
(#3051, the `resolveMoveFanoutColumnsSync` family, #3114, and my own
`--triage` draft that failed to zero).

## What this does

One line: adds `node scripts/check-inert-sync-lane-conversions.mjs` to
`test:gate`, beside the cheap AST guards it belongs with
(`check-no-nohup`, `check-capacity-pool-id`, `check-mock-completeness`).
Single-pass parse; no measurable cost.

## Scope reduced from the earlier version

This branch previously also restored the triage literal. **#3126 landed
that first**, so I rebuilt it as the wiring alone rather than carry a
half-redundant diff. #3127 is closed with nothing lost.

## Census before / after

```
before:  COLUMN guards (the backlog):   18
after:   COLUMN guards (the backlog):   18
```

Unchanged — this converts nothing. It makes an existing check actually
run.

## Verification

`test:gate` exit 0 **with the ratchet inside it**, against current
`main` (now green on the ratchet since #3126). Diff is one line of
`package.json`.

## Related

**#3122** restores 13 guards laundered through `mergeParkedColumns` —
worth landing after this so the ratchet enters CI at full sensitivity.
**#3135** attaches lanes at the last two live emitters. **#3131**
corrects a 44% under-report in `--triage`.

package.json

@@ -29,7 +29,7 @@
     "census:lifecycle-columns": "node scripts/lifecycle-column-census.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=$!; pnpm --filter @fusion/core test:pg-gate & pg_pid=$!; 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 $status' && pnpm --filter @runfusion/fusion test:ci-shape",
+    "test:gate": "node scripts/check-no-nohup.mjs && node scripts/check-no-cwd-relative-dashboard-test-reads.mjs && node scripts/check-no-kill-4040.mjs && node scripts/check-no-getdatabase.mjs && node scripts/check-capacity-pool-id.mjs && node scripts/check-no-node-only-core-imports-in-dashboard.mjs && node scripts/check-pi-versions-pinned.mjs && node scripts/check-no-test-timeout-appeasement.mjs && node scripts/check-changeset-format.mjs && node scripts/check-mock-completeness.mjs && node scripts/check-inert-sync-lane-conversions.mjs && sh -c 'pnpm --filter @fusion/engine test:core & engine_pid=$!; 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",