fix(@angular/build): coalesce external watcher directories to avoid macOS FSEvents limit

Previously, `ParcelExternalManager` immediately created a separate `@parcel/watcher` subscription for the parent directory of every watched file outside the workspace root. In monorepo setups where an application imports from hundreds of external library directories and hoisted `node_modules` packages, this exceeded macOS's ~512 per-process `FSEvents` stream limit, causing subsequent subscriptions to silently fail and miss file changes.

To prevent exhausting native watcher streams, `ParcelExternalManager` now normalizes external `node_modules` file paths to their package root directory and batches synchronous file additions and removals within a microtask so nested child directories are subsumed before subscribing. When the number of external target directories exceeds the subscription budget of 256, a prefix tree is used to iteratively collapse the deepest branching subdirectories into common ancestors while preferring directories that do not contain `workspaceRoot`. Additionally, if `parcelWatcher.subscribe()` rejects for an existing directory on disk, the manager dynamically lowers its subscription budget and re-reconciles into higher-level ancestor directories.

Closes #34246

Author: clydin