fix: platform-specific watcher for macOS FSEvents + Windows RDCW

macOS: raw notify with custom 50ms debounce thread. FSEvents provides
complete event kinds (Create/Remove/Modify) without needing a file
cache, avoiding the debouncer-full's Create+Remove cancellation bug.

Windows/Linux: notify-debouncer-full with RecommendedCache. RDCW needs
the file cache to detect cross-directory moves and renames. The cache
also handles RDCW's limited event granularity.

Both platforms share the same convert_event logic for mapping notify
EventKinds to VfsEvents.

All 489 tests passing locally (macOS).

Cargo.lock

@@ -1030,6 +1030,15 @@ version = "2.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
 
+[[package]]
+name = "file-id"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1fc6a637b6dc58414714eddd9170ff187ecb0933d4c7024d1abbd23a3cc26e9"
+dependencies = [
+ "windows-sys 0.60.2",
+]
+
 [[package]]
 name = "filetime"
 version = "0.2.27"
@@ -1911,6 +1920,7 @@ dependencies = [
  "fs-err",
  "log",
  "notify",
+ "notify-debouncer-full",
  "serde",
  "tempfile",
 ]
@@ -1980,6 +1990,19 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "notify-debouncer-full"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c02b49179cfebc9932238d04d6079912d26de0379328872846118a0fa0dbb302"
+dependencies = [
+ "file-id",
+ "log",
+ "notify",
+ "notify-types",
+ "walkdir",
+]
+
 [[package]]
 name = "notify-types"
 version = "2.1.0"