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

[clydin]

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

[gemini-code-assist]

Code Review

This pull request refactors the ParcelExternalManager in the Angular build package to optimize directory subscriptions when using @parcel/watcher. It introduces a directory prefix tree (trie) to coalesce multiple external directory subscriptions into common ancestors when the subscription count exceeds a budget (defaulting to 256), preventing macOS FSEvents stream limit errors. Additionally, it adds comprehensive unit tests to verify batching, path normalization, coalescing logic, and dynamic budget reduction. There are no review comments, and the changes look solid and well-tested.