feat(watcher): enable rspack native watcher

[stormslowly]

Why

Rstest uses the JS file watcher in watch mode because rspack's native (Rust) watcher had a macOS flake — stale FSEvents replayed an empty rerun (expected '' to match 'other.test.ts'), failing ~7.5% on macos-14 runners.

That root cause is fixed upstream and shipped in rspack 2.1 (safeTime / FS_ACCURACY granularity-safe scan + ignored-subtree event filtering). main already runs on released @rsbuild/core ~2.1.4 / @rspack/core ~2.1.2, so the fix is already in place — this PR only flips the watcher on, no version bump.

Validation: the native watcher was stress-tested on macos-14 (node 22/24, path-reuse condition) for 700+ watch runs across the upstream fix series — zero watcher flake. The released 2.1.x line is a superset of those builds; local macOS re-run on 2.1.4 is 10/10 green.

What

• Enable experiments.nativeWatcher in watch mode.
• Raise the default watch aggregateTimeout 5 → 100ms — on macOS the native watcher needs the FSEvent → vnode cache invalidation window to elapse before rspack stats the changed file, otherwise the rebuild reads stale content. Drop the now-redundant per-fixture aggregateTimeout: 10 overrides in the watch e2e configs so this default applies.
• Make config-file watching reliable on macOS: poll the small config-file set (chokidar v5 dropped fsevents and fs.watch silently drops single-file change events on macOS) and await the initial scan so the first change is never missed.

[stormslowly]

Held as draft — blocked on an upstream rspack 2.1 source-map regression.

CI fails on 3 non-watcher e2e tests (list/index, list/json, test-api/timeout) that assert includeTaskLocation source columns. rspack 2.1.0-beta.0 changed the source-map mapping for ESM interop member-call expressions:

(0, _core.describe)('test a', ...)   // bundled

The call-site column now resolves to the source ( instead of the describe/it identifier:

location	rspack 2.0.x	rspack 2.1.0-beta.0
describe('test a' (L3)	3:1	3:9
it('test a-1' (L4)	4:3	4:5
it('test a-2' (L9)	9:1	9:3

Root cause confirmed via minimal repro + source-map decode. Not configurable (tested experiments.sourceImport, optimization.concatenateModules, devtool, output.column), not yet fixed on rspack main, and rspack 2.1 is required for the native-watcher fix (so downgrading is not an option).

Holding this PR until the upstream regression is fixed; the task locations will then snap back without any test changes here.