feat(core): virtual file entry and native watcher for watch mode

[stormslowly]

Why

In watch mode the CLI "rerun all" shortcut re-ran tests without recompiling, so newly added test files were never picked up (the old // TODO: should rerun compile with new entries).

This PR drives every rerun through Rspack's compile pipeline by writing to a virtual entry module, and switches watch mode to Rspack's native (Rust) file watcher.

What

• Virtual file entry: register a per-environment VirtualModulesPlugin entry. triggerRerun() rewrites it with a fresh value to force a recompile, and the recompile drives the test run through the normal onAfterDevCompile path.
• Route the "rerun all" shortcut through triggerRerun() + forceRerunOnce so it recompiles with the current entries instead of re-running the stale build.
• Native watcher: enable experiments.nativeWatcher in watch mode and lower watchOptions.aggregateTimeout to 5ms for snappier reruns.
• e2e coverage for watch / restart / shortcuts.

Verification

Locally verified the native watcher end-to-end on examples/node:

step	result
initial run	3 passed
edit a source dependency (src/index.ts)	rerun within ~0.5s, test flips to failed
revert	back to 3 passed

Runtime-confirmed the flag selects the native FS: experiments.nativeWatcher: true → NativeWatchFileSystem, false → NodeWatchFileSystem (rspack 1.5.0).

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