Load built host artifacts natively in tests

[ymichael]

Human comments

What was wrong

builtin-host-artifacts.test.ts builds final, self-contained host.js files and then dynamically imports them inside Vitest. The package left those generated artifacts inline in Vitest's module runner, so Vite parsed and transformed the already-built provider bundles again instead of handing them to Node's native loader as the daemon does. Under package-shard CPU oversubscription, that avoidable second pass stretched the Keep Awake case past its 20-second budget; Vitest timed the case out without cancelling its in-flight import, which then continued into later work. The independently verified merge base for this fix is 524b2fe4b0df74185bdd29a274fb77cd6765b2b3.

What changed

• Externalize only .builtin-host-test-*/dist/host.js in @bb/plugin-build's Vitest configuration so the final ESM artifact is loaded natively.
• Keep all five real artifact builds, dynamic imports, handler/bridge/contract assertions, and the existing timeout ceilings unchanged.
• No production behavior, wire contract, CLI, guide, or daemon protocol changes.

How you verified

• Same bounded contention harness before and after (two groups of 512 CPU workers, exact Keep Awake test): before timed out at 20.006s; after passed twice at 12.559s and 16.073s. Instrumentation isolated the changed phase: host build was effectively unchanged (11.277s before, 11.057s after), while artifact loading fell from 12.157s to 0.844s. The pre-fix import and handler logs arrived after Vitest had already reported the timeout, confirming the uncancelled overlap.
• Unloaded full artifact file: 13.22s before to 2.64s after; aggregate Vitest transform time fell from 6.72s to 136ms. All five existing assertions passed.
• TMPDIR=/private/tmp pnpm exec turbo run test --filter=@bb/plugin-build --force --env-mode=loose — 10 files passed, 134 tests passed, 1 skipped.
• TMPDIR=/private/tmp pnpm exec turbo run test --filter=@bb/plugin-build --force --env-mode=loose -- --coverage — same tests passed with V8 coverage enabled.
• pnpm exec turbo run typecheck --filter=@bb/plugin-build --force — 4/4 tasks passed.
• pnpm exec turbo run build --filter=@get-bb/plugin-sdk --force — 2/2 tasks passed; 17 runtime entries built.
• pnpm exec oxfmt packages/plugin-build/vitest.config.ts --check.

AGENT GENERATED: by GPT-5.6-Sol