fix(watch): prevent native watcher handle leaks

[sunerpy]

Summary

• use one recursive root watch on Windows and macOS instead of one native watcher per directory
• keep the existing pruned per-directory non-recursive strategy on Linux and other targets
• stop re-registering newly created directories on native-recursive backends
• add strategy tests and a Windows handle-count regression test for delete/recreate cycles

Root cause

notify 6 uses the natively recursive ReadDirectoryChangesW backend on Windows. CodeGraph registered every directory separately and registered recreated paths again. Each registration owns a 16 KiB buffer and OS handles; replacing an existing path in notify leaks the previous watch state, so long-running Zed/OpenCode sessions grow monotonically.

Verification

• make ci
• pre-push fmt, clippy, workspace tests, golden/schema parity, and guardrail
• Windows-native regression runs in the Windows CI job

Scope

This is the watcher-only P0 hotfix. Windows daemon pipe/proxy lifecycle changes remain separate because they require coordinated watchdog and half-close fixes.

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/codegraph-watch/src/watcher.rs	97.36%	2 Missing ⚠️

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   95.26%   95.26%           
=======================================
  Files         124      124           
  Lines       61191    61261   +70     
=======================================
+ Hits        58295    58362   +67     
- Misses       2896     2899    +3     

Files with missing lines	Coverage Δ
crates/codegraph-watch/src/watcher.rs	94.24% <97.36%> (+0.21%)	⬆️

... and 2 files with indirect coverage changes

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