Fix build mode setup stall on large solutions

[johnfav03]

Fixes #4614

On a large tsc -b --watch project-references solution, Found 0 errors. Watching for file changes prints within a few seconds, but filesystem watches aren't registers until computeDesiredWatches returns, which can take tens of seconds. Edits are also silently dropped during this timeframe.

computeDesiredWatches calls IsDirCoveredByWatch once per input file, buildinfo file, and package.json ancestor across all projects, which linearly scans the entire DesiredDirs map on every call. The set construction here is essentially O(files x dirs); by using a new data structure called DirWatchSet, we can reduce this to O(files x recursive dirs). In the case of the example given in the linked issue, the ~1,534 desired dirs drop to ~15 recursive dirs while the number of files remains the same.

[Copilot]

Pull request overview

Optimizes watch setup for large build-mode solutions by replacing repeated full directory-map scans with indexed coverage checks.

Changes:

• Adds DirWatchSet with constant-time exact lookup and recursive-watch scanning.
• Integrates it into build-mode and standard watcher setup.
• Adds coverage, casing, and recursive-upgrade unit tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/execute/watchmanager/watchmanager.go	Implements DirWatchSet.
internal/execute/watchmanager/watchmanager_test.go	Tests watch-set behavior.
internal/execute/watcher.go	Uses indexed coverage for standard watch setup.
internal/execute/build/orchestrator.go	Optimizes build-mode watch computation.