Represent special filesystem path types

[lukewilliamboswell]

Summary

• replace the host path-type boolean record with an exhaustive ABI tag union
• expose IsOther from Path.type! so sockets, FIFOs, devices, and unrecognized Windows reparse points are never reported as regular files
• document predicate behavior and cover regular files, directories, symlinks, FIFOs, and Unix-domain sockets

Testing

• cargo test --lib
• ./scripts/test.py
• generated Rust glue regeneration check using the latest nightly compiler and matching glue spec

The release bump check was attempted against 0.21.0-rc4, but the old package no longer compiles with the current compiler because it references Json.ParseErr. A comparison against current main is also blocked by the existing InternalHttp.TransportErr public-API extraction error. This PR still expands the public Path.type! tag union, so the release bump workflow will detect the API change once those existing compatibility blockers are resolved.

Fixes #441