fix(tests): serialize package builds across parallel vitest workers

[arham766]

Summary

Fixes #3479. Three test files build into packages/core/dist from beforeAll: scripts/build-css.test.mjs runs a full pnpm build unconditionally, and the two build-theme test files build core when dist/theme/index.js is missing. Vitest runs test files in parallel worker processes, so two of those builds can run concurrently over the same dist directory: one build's rimraf dist races the other's output, failing with ENOTEMPTY: rmdir dist/utils or esbuild: Could not resolve dist/index.js. That is exactly the failure on the CI run for #3466 (a PR whose diff touches none of these files; adding a test file reshuffled vitest's worker distribution enough to expose the race).

The fix adds scripts/repo-build-lock.mjs: a withRepoBuildLock(fn) helper that serializes the builds with an atomic lock directory (fs.mkdirSync either creates it or throws EEXIST) and a bounded wait with a stale-lock hint. All three files run their build under the lock, and the build-theme files re-check existsSync inside it, so a waiter skips the rebuild the holder already completed. The lock directory is gitignored.

Test plan

• Mutual-exclusion stress: 6 concurrent node processes each acquire the lock and write/hold/remove a marker file that fails the run if a second holder ever enters. All 6 serialize cleanly; the lock directory is released at exit.
• Normal path: with core built, both build-theme files run through the lock (acquire, skip build, release) and their tests behave exactly as before. build-theme.prose.test.mjs passes 2/2 locally; build-theme.import-path.test.mjs is 4/5 with the one failure being the pre-existing Windows path-separator issue tracked in [Bug] Test suite fails on Windows due to path separator and URL.pathname issues #3233 (present on main before this change).
• The race path itself (two workers both seeing dist missing) cannot be exercised faithfully on this Windows machine because execFileSync('pnpm', ...) cannot spawn pnpm.cmd there (also pre-existing, part of [Bug] Test suite fails on Windows due to path separator and URL.pathname issues #3233), so the concurrency correctness is covered by the stress test above plus Linux CI on this PR.
• node scripts/check-changesets.mjs passes.

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