test: Add regression tests and OutputWatcher trait to prepare for daemon removal from turbo watch (#12171)
Summary

Defines the OutputWatcher trait that will replace DaemonClient in
RunCache/TaskCache when we remove the daemon from watch mode
Adds regression tests that characterize current watch behavior and
must stay green throughout the daemon removal refactor

What’s here
OutputWatcher trait (turborepo-run-cache): Abstracts
get_changed_outputs and notify_outputs_written — the two operations
TaskCache uses to prevent infinite rebuild loops in watch mode.
Currently these go through the daemon’s gRPC interface; the trait lets
us swap in an in-process GlobWatcher without changing TaskCache.
8 mock-based unit tests (turborepo-run-cache): Characterize the
trait contract — empty results, partial changes, error recovery for both
get and notify, lifecycle round-trips, per-hash independence, and `Send

Sync` compliance.

3 GlobWatcher delegation tests (turborepo-filewatch): Verify that
GlobWatcher::watch_globs/get_changed_globs satisfy the
OutputWatcher contract when called with raw strings and
GlobSet::from_raw — the exact path the future InProcessOutputWatcher
will use. Tests cover unchanged outputs, changed outputs, and exclusion
glob handling.
2 E2E watch tests (watch_test.rs):

watch_no_spurious_rebuild_after_settle — After the initial build, no
further rebuilds occur when no files change. Guards against infinite
rebuild loops from cache restores triggering the file watcher.
watch_same_content_write_does_not_rebuild — Writing identical
content to a source file does not trigger a rebuild. Guards against
hash-based deduplication regression.

Testing
All 13 new tests pass locally. The 2 E2E tests are automatically
serialized via the existing nextest turbo-watch-serial group.