perf(test): run the unit project with isolate:false (#2647)

Follow-up to #2644. With `isolate: true` every test file re-executes its
module graph in a fresh registry; that is required for the `dom` projects
(they leak ComponentRegistry overrides and happy-dom nodes across files)
but not for `unit`, which is node-env pure logic with no such shared
mutable state.

Relaxing isolation just for `unit` lets the worker share one module graph
across its files. Measured 3.2x faster for the project in isolation
(38s -> 12s), which is a real speedup for local `vitest run --project
unit` and watch-mode logic TDD. Full-suite / CI wall barely moves (the
`dom` project gates it and runs concurrently), so this is a developer-
experience win, not a CI one — the CI win needs the DOM hermeticity work
tracked separately.

Verified hermetic, not luckily-ordered: 241 files / 3215 tests stay green
across a plain run plus three `--sequence.shuffle` seeds (1, 42, 98765),
and the full suite is unchanged at 566 files / 6894 tests.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: os-zhuang

vitest.config.mts

@@ -126,6 +126,15 @@ export default defineConfig({
         test: {
           name: 'unit',
           environment: 'node',
+          // The unit project is node-env pure logic with no ComponentRegistry
+          // or DOM state to leak across files, so it can share a module graph
+          // per worker instead of re-executing it per file. Measured 3.2x
+          // faster (38s -> 12s for the project) with zero failures, holding
+          // green across repeated and shuffled runs. The `dom`/`dom-heavy`
+          // projects keep `isolate: true` — they DO leak (registry overrides,
+          // happy-dom nodes); relaxing them needs the hermeticity fixes tracked
+          // separately.
+          isolate: false,
           setupFiles: [path.resolve(__dirname, 'vitest.setup.base.ts')],
           include: ['packages/**/*.test.ts', 'examples/**/*.test.ts'],
           exclude: [...sharedExclude, ...domTsTests],