perf(cascade): relativise watcher paths without a resolve() per event

[gloryfromca]

Summary

_relative_to_root in the cascade watcher called Path(raw).resolve() on every watchdog event (create / modify / move / delete) to defend against an external symlink surfacing inside the tree. watchdog already hands us absolute paths inside the watched root, so in the common case that is a filesystem round trip per path component for nothing — on Windows a GetFinalPathNameByHandle call per component, each through Defender.

The textual relative_to now runs first; resolve() is kept only for a path that is not under the root as written, or that carries .. segments. Same results for the symlink case (still maps) and for outside paths (still None).

Evidence

py-spy, 25 s, the soak server under raised write load (three feeders ≈ 30 md rewrites/s + two cascade sync storms + a search storm, PR #454's box): of 1 082 worker-thread samples, 435 (40 %) were in realpath / _getfinalpathname_nonstrict / stat, and 403 of those (93 %) came from _relative_to_root via _Handler._enqueue (on_modified 221, on_created 72, on_moved 91, on_deleted 19). The Python-level ntpath/pathlib frames around those calls hold the GIL; the Win32 calls themselves release it.

Area

• performance (cascade / Windows)

Verification

• tests/unit/test_memory/test_cascade: 221 passed.
• New test_relative_to_root_within_does_not_touch_the_filesystem: Path.resolve patched to raise; red without the fast path (AssertionError: resolve() must not run for an in-root path), green with it.
• New test_relative_to_root_via_symlink_still_resolves (POSIX): a path reaching the root through an external symlink still maps to the same relative path.
• make lint green.

Checklist

• tests added, mutation checked
• behaviour unchanged for symlink / outside / .. inputs

Notes for Reviewers

root is MemoryRoot.root as configured; if it is itself given through a symlink or a different case than watchdog reports, the textual relative_to fails and the old resolve() path runs — so the fallback is exercised, not lost. Windows PureWindowsPath.relative_to compares case-insensitively.

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