ci: run PR unit shards on free hosted ARM as 12 shards (#34906)

* ci: run PR unit shards on free hosted ARM as 12 shards

Enterprise's 1,000-job limit makes more, smaller shards free. A 12-shard
cold run on ubuntu-24.04-arm measured 361s median / 412s max per shard,
against ~390s / ~441s for five warm Blacksmith 8-vCPU shards.

The Blacksmith-store module cache producer becomes dispatch-only since
hosted shards cannot read it.

* TEMP: force 12 shards on this PR (repo var is still 5); drop before merge

* test: size the full-inventory routing check from FULL_SHARD_COUNT

* Revert TEMP 12-shard override; the repository variable is now 12

---------

Co-authored-by: OrcaWin <293788423+OrcaWin@users.noreply.github.com>

.github/workflows/unit-module-cache.yml

@@ -1,29 +1,9 @@
 name: Unit transform cache producer
 
-# Cold full unit run on main that publishes per-shard Vitest transform caches for PRs.
+# Cold full unit run on main that publishes per-shard Vitest transform caches.
+# Dispatch only: PR shards run on hosted runners, which cannot read this Blacksmith-store cache.
 on:
-  schedule:
-    - cron: '17 */6 * * *'
   workflow_dispatch:
-  push:
-    branches: [main]
-    # Cache-key inputs (see ci-unit-module-cache.mjs) change the key; reseed right away.
-    paths:
-      - '.github/workflows/unit-module-cache.yml'
-      - '.github/workflows/unit-tests.yml'
-      - 'config/vitest.config.ts'
-      - 'config/scripts/vitest*'
-      - 'config/scripts/ci-unit-module-cache*.mjs'
-      - 'config/patches/**'
-      - 'config/scripts/workspace-source-exports.mjs'
-      - 'config/dark-build-switches.cjs'
-      - 'config/pnpmfile.cjs'
-      - '.npmrc'
-      - '**/package.json'
-      - '**/tsconfig*.json'
-      - 'pnpm-lock.yaml'
-      - 'pnpm-workspace.yaml'
-      - 'config/.bun-version'
 
 concurrency:
   group: unit-module-cache-${{ github.ref }}