## Why
Special filesystem subpaths can contain relative or foreign-platform path
spellings. Treating this wire text as a host-native path can reinterpret it.
## What changed
- Store special filesystem subpaths as strings in the core permissions model.
- Represent app-server subpaths as `LegacyAppPathString` and convert them at
the protocol boundary without changing their spelling.
- Add `LegacyAppPathString::from_string` for callers that already own legacy
app-server path text.
GitOrigin-RevId: 2214f03aa997fd0e6eb2ab60d425304bb4bd6132