feat(kathodos): artist alias symlink management (#162)

[forkwright]

Summary

• Adds crates/kathodos/src/alias.rs with four public functions: create_artist_alias, remove_artist_alias, list_artist_aliases, and resolve_artist
• Symlinks are relative (canonical name only), living alongside canonical directories under library_root
• Alias names pass through the same sanitize_path_segment pipeline as canonical names
• Creating an alias over an existing symlink updates the target; creating over a real directory is an error

Test plan

• create_artist_alias — symlink exists and points to canonical
• remove_artist_alias — symlink gone after removal
• list_artist_aliases — returns all symlinks pointing to canonical, sorted; excludes unrelated aliases
• resolve_artist canonical name — returns the path itself
• resolve_artist alias — follows symlink, returns canonical path
• Error: alias name conflicts with real directory
• Update: creating alias over existing symlink updates target
• Error: remove_artist_alias on non-symlink path
• Error: create_artist_alias when canonical does not exist

All 77 cargo test -p kathodos tests pass. cargo check --workspace is clean.

Closes #162