feat(watcher): enable rspack native watcher

[stormslowly]

Why

Rstest has been using the JS file watcher in watch mode because the rspack 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 now fixed upstream in rspack 2.1.0-beta.0:

• safeTime / FS_ACCURACY granularity-safe scan (feat(watcher): granularity-safe native watcher scan (watchpack FS_ACCURACY)  rspack#14306)
• ignored-subtree event filtering (fix(watcher): filter ignored-subtree events that emit a spurious folder change rspack#14329)

With the fix shipped, rstest can switch to the native watcher.

Validation: native watcher on rsbuild 2.1.0-beta.0 was stress-tested on macos-14 (node 22/24, path-reuse condition) — 80/80 green, zero watcher flake.

What

• Upgrade @rsbuild/core / @rspack/core to 2.1.0-beta.0 across the workspace.
• 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.
• adapter-rspack: drop the top-level snapshot mapping — snapshot was removed from RspackOptions in rspack 2.1 (it now lives under persistent cache).

[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.