perf(windows): consolidate watch paths via ConsolidatingPathTrie

Close #62.

Mirror the fsevents PR #60 pattern: when 10+ sibling paths are registered
under the same parent, collapse them into a single watch on the parent
instead of opening one `ReadDirectoryChangesW` handle per child.

Implementation:
* Replace the `WatchMode` value of `self.watches` with a new `UserWatch`
  struct that carries the user's intent plus the resolved OS-level
  primary dir (and optional tracked-parent for `TargetMode::TrackPath`).
* Add `rebuild_watch_handles`: build a `ConsolidatingPathTrie` over the
  primary dirs from every user watch, compute the recursive flag per
  consolidated target by checking for strict descendants in the user
  watches, layer in tracked-parent watches that aren't already covered,
  and diff against the open `watch_handles` to converge the OS state.
* `add_watch` / `remove_watch` / `apply_staged` now update `self.watches`
  first and then call `rebuild_watch_handles` instead of poking
  `add_watch_raw` directly.
* The event filter at `handle_event` was a direct hash lookup on the OS
  dir name; consolidation can leave the OS dir different from any user
  request, so it now walks ancestors via `is_event_covered`.
* `pre_open_tracked_parent` works around a Windows quirk where calling
  `path.metadata()` on a directory *before* its parent dir has a
  `ReadDirectoryChangesW` handle silently drops the parent's later
  `FILE_ACTION_MODIFIED` event for that directory. The pre-call opens
  the tracked parent before any metadata is read, matching the order
  the previous per-call code used.

Adds five new tests covering: 10-sibling consolidation, event delivery
through a consolidated parent, mixed recursive/non-recursive collapse,
de-consolidation when the count drops back below the threshold, and the
batched `paths_mut().commit()` path.

Author: sapphi-red