test(browser): isolate fixture optimizer caches

[wesbillman]

Summary

Small prerequisite split from #39; no production changes.

• Give nine existing Vite fixture consumers an individually owned temporary optimizer cache.
• Preserve cleanup through listen/page setup and earlier teardown failures.
• Keep browser assertions, timing budgets and zero retries unchanged.
• Make the existing contributor scope-growth checkpoint explicit.

Why

PR39's failed WebKit job
left the terminal fixture blank after three module imports returned 504 Outdated Optimize Dep.
These fixture servers shared Vite's default optimizer directory. This fixes cache ownership;
it does not add retries or suppress module errors. It is independent of presence implementation.

Validation

At 4ec53255a41498caa7e8bec705e252a7a219b929, all 64 Chromium/WebKit journeys
in the nine changed fixture files pass with two workers and no retries.
Checks/type checks and the three Node cache-ownership tests passed on the exact extracted patch.
Independent review verified extraction against the reviewed combined patch.

On an isolated copy of that combined patch, twelve mutations were caught: shared-cache reuse,
lost cleanup, and bypassing the helper in each of ten consumers (the extra consumer is
presence.spec.mjs, which stays in #39). This proves ownership regression detection, not that
we deterministically reproduced which CI server invalidated which optimized import.

Hosted CI and required approval remain separate gates. No native/runtime production change.

Origin: presence-sensing channel 2128346d-ae35-4e44-98f7-5f278fe7b60d.

[wesbillman]

Carl, an automated reviewer, commenting via Wes’s GitHub account.

No material code findings at abc257755f27ee944c4de2fa7de449dc20d150d7, against 61274d40e0171cae4ef519021370345e18b884d3.

• Reviewed the cache helper, all nine consumer migrations, setup/teardown ownership, test discovery and the observer-test clock freeze. Unique cache ownership is enforced at creation; cleanup follows Vite shutdown and survives earlier teardown failures. Browser assertions, timing budgets, retries and production behavior are unchanged.
• Exact-head hosted checks are green, including both browser engines and Node/Vitest coverage. Independent source-review lanes found no material regressions. No local suites were rerun. The reported 64-journey local run belongs to the cache commit 4ec53255; the later head adds only the test clock freeze.
• Validation boundary: this is not an attestation of a full local just scan. The PR body reports focused local validation, not that full gate; the three documented local-only WebKit cases remain outside hosted CI. Keep the repository's remaining local-gate requirement or explicit owner exception separate from this code verdict. Required human approval is also separate; this comment is not an approval.