docs, ci: apply second serial-linux review pass

Address the review findings from the fresh pass on PR #2744:

- 2026-07-21 note L31: the enabled serial references run on the self-hosted
  vm-backup/dsh-win-ci pools and the only remaining disabled hosted serial is
  serial-macos (macos-latest); removed the stale hosted ubuntu-latest/windows-2025
  serial framing and the outdated 'when enabled, serial / windows' clause.
- Drop remaining change-narrative from both notes: L19 (serial / linux, macos)
  and the 2026-07-26 caching note L16/L34 now state only current facts without
  deletion dates/PR numbers; the no-producer fact has one home (L16). zh.ms'
  '直至其过期为逐出' corrected to '直至其被逐出'. Bilingual hashes re-recorded.
- ci.yml TODO notes that re-enabling serial-macos does not restore a Linux
  hosted-cache producer and records the seeder-vs-remove decision direction.
- The Playwright restore's failover-skip comment is now self-contained (the
  VM's persistent browser cache is warm) instead of pointing at the coveragelane
  rationale, which is pnpm-store-specific.

Verification: scripts/ci-workflow.spec.ts passes (12/12), YAML re-parses,
both translation pairs consistent, git diff --check clean.

.github/workflows/ci.yml

@@ -44,7 +44,9 @@ jobs:
 
   # TODO(hosted-serial-ci): Re-enable the one remaining disabled hosted serial
   # reference job (serial-macos) before release. The self-hosted standby lane
-  # below remains active on every master push.
+  # below remains active on every master push. Re-enabling serial-macos does not
+  # restore a Linux hosted-cache producer: decide whether to add a master seeder
+  # or remove the restore-only steps if cold starts become a concern.
 
   # Three enterprise jobs isolate coverage, static analysis, and the
   # build-backed consumer tail. The consumer job owns the only Linux build so
@@ -225,7 +227,7 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-
 
-      # Skipped under failover — see the coverage lane's identical rationale.
+      # Skipped under failover: the VM's persistent browser cache is already warm.
       - uses: actions/cache/restore@v4
         if: vars.DSH_CI_FAILOVER_LINUX != 'selfhosted' || github.event.pull_request.user.login == 'dependabot[bot]'
         with: