glob: add shared result path type

Summary:
Add `GlobPath`, a compact path representation that shares each directory prefix and stores only the basename per result. Comparisons and Thrift serialization operate directly on the pieces without materializing the full path. Paths converted before shared traversal state is available retain one owned full-path buffer, avoiding separate prefix and basename allocations. `toIOBuf()` returns its head wrapper by value to avoid one temporary allocation per serialization call.

This also adds `PathComponent::intoStorage()` so traversal can move `PathComponent` storage into result objects without another basename allocation. This commit introduces and tests the representation; the following commit wires it into glob traversal and Thrift responses.

Reviewed By: genevievehelsel

Differential Revision: D113152048

fbshipit-source-id: a761dc3c447150f9800c1f39b667717794a3ff53

Author: muirdm

build/fbcode_builder/manifests/watchman

@@ -30,7 +30,7 @@ fbcode/watchman/oss = .
 fbcode/eden/fs = eden/fs
 
 [shipit.strip]
-^fbcode/eden/fs/(?!.*\.thrift|service/shipit_test_file\.txt)
+^fbcode/eden/fs/(?!.*\.thrift|service/shipit_test_file\.txt|utils/GlobPath\.h$)
 
 [cmake.defines.fb=on]
 ENABLE_EDEN_SUPPORT=ON