Share vitest workers for node-environment tests and cut fixed per-file costs - #2303

What was wrong

The test suites spent most of their time outside the tests. Profiling every package uncontended showed that 80–90% of the big suites' CPU was per-file overhead: vitest re-imported the whole module graph, built jsdom, and ran the setup file for each of @bb/app's 422 files; the setup file's @bb/shared-ui/icon-extended import pulled @hugeicons/core-free-icons, a barrel that re-exports 5,122 one-icon modules and takes ~0.7 s to load; and every @bb/server test harness replayed all 107 SQLite migrations (57 of the 61 ms a harness costs, paid by 1,922 tests). @bb/server and @bb/agent-runtime already set isolate: false, but vitest 4 only hands a finished worker the next queued file when that file has the same project and environment, so the interleaved queue still churned workers.

Per-test profiling (JSON reporter) then showed fixed costs inside the slowest suites: the templates external-scaffold test ran four cold npm installs of the packed SDK (~13 s each); db data suites migrated per test (~17 s of their 30 s); every integration harness rebuilt the first-party provider bridges with esbuild (~0.6 s); provider-parity's 43 replay cells are bridge children that mostly wait on settle/drain pacing, so the suite was wall-clock bound by vitest's default maxConcurrency of 5; and two Pi SDK tests slept through eight real 250 ms retry delays each.

What changed

• vitest.shared.ts exports sharedWorkerProjects(). It scans a package's test files and splits them into shared-worker projects (isolate: false, one per non-DOM environment) and an isolated project for files that mutate worker-global state — vi.mock/vi.stubGlobal/vi.stubEnv/vi.resetModules, process.env writes, process.chdir, assignments to window/globalThis/document/navigator/prototypes (including through a cast), Object.defineProperty/Reflect.* on them — directly or through a test helper they import. Files that run in jsdom/happy-dom stay isolated: pushing the app's jsdom files through one worker failed a different file on every ordering (portals, focus, media-query caches keyed by the shared document), which no source scan can enumerate. Every project keeps the package's own include globs and excludes the other projects' files, so the split never changes which files run.
• SharedWorkerSequencer (installed by defineWorkspaceTestConfig and the root config) orders the run queue: isolated files first, then each shared project contiguously. This is what turns isolate: false into worker reuse without groupOrder phase barriers, which left workers idle at each phase tail.
• Tests alias the bare @hugeicons/core-free-icons specifier to the package's self-contained minified bundle (identical exports; 45 ms instead of ~700 ms per worker).
• 23 package vitest.config.ts files use the helper; @bb/server and @bb/agent-runtime drop their hand-rolled split.
• apps/server/test/helpers/test-app.ts adds createTestDb(), which migrates once per worker and opens each harness from the serialized image; createConnection accepts a Buffer for that. packages/db/test/helpers/migrated-connection.ts does the same for the db data suites (suites that exercise migrate itself still call it directly).
• packages/templates/test/plugin-scaffold-external.test.ts installs the packed SDK once per file and symlinks each scaffold's node_modules at it, passes --no-audit --no-fund --prefer-offline, and overlaps the backend and frontend typechecks.
• apps/server/test/helpers/provider-registry.ts memoizes the first-party bridge artifact build per worker process.
• packages/provider-parity/vitest.config.ts sets maxConcurrency: 16 (measured pinned to 4 CPUs: 37 s at 5, 21 s at 10, 15 s at 16, all green).
• Two Pi SDK retry tests drive the retry delay with fake timers.
• internal-skill-trees.test.ts pins the fixture's file mode (it failed under umask 002), @bb/qa gets a 15 s test timeout, and AGENTS.md documents the convention.

How you verified

Clean, uncontended runs on a 16-core machine: @bb/app 63.0 s → 38.6 s, @bb/server 47.6 s → 32.0 s, @bb/mobile 4.4 s → 2.4 s, @bb/provider-parity 36.5 s → 14.0 s, @bb/integration-tests 24.8 s → 16.6 s, @bb/db 7.5 s → 3.5 s, the templates external test 27.3 s → 23.5 s with a warm npm cache (three cold installs fewer in CI), sdk-session.test.ts 4.0 s → 0.3 s; other packages at parity or slightly faster. pnpm exec turbo run test --force --continue 3 m 27 s → 2 m 22 s with everything contending, 71 of 72 tasks passing. The one failure, @bb/qa's "starts a detached daemon repeatedly" test, times out on the development machine before and after this change. Stress: every shared project run through a single worker (--project <name> --maxWorkers=1) passes for app (149 files), server (184), tasks, host-daemon, and cli; vitest run --shard=1/3 works for the app. pnpm exec turbo run typecheck passes for db, server, app, templates, integration-tests, provider-parity, and agent-runtime; lint passes; tsc --noEmit passes on vitest.shared.ts.

No linked issue.

AGENT GENERATED