uv-fs-stub

Minimal local stub for the uv-fs crate from uv, stripped to only the functions needed by uv-pep508 and uv-requirements-txt.

normalize_url_path -- percent-decodes and normalizes a URL path component for use as a file path
normalize_absolute_path -- normalizes an absolute path by resolving . and .. components
Simplified trait -- simplified path display (delegates to .display() in WASM); used by uv-requirements-txt for error messages
read_to_string_transcode -- reads a file via host-bridge read_file; used by uv-requirements-txt to read included -r/-c files

Everything else (async I/O, tokio, filesystem utilities, symlink helpers, etc.) has been removed. The tokio and serde features are kept as no-ops so that dependents enabling them still compile.

Based on uv revision 35d1e90.