feat(paths): implement scope-based path display resolution

[botirk38]

Summary

Implements proper path display resolution based on user-provided search scopes, addressing the "Default path printing" partial row in the rg compatibility matrix.

• Add PathDisplay enum (Relative/Absolute) to control path output format
• Refactor effective_path_display() in CLI to use user-provided scopes only
• Update core to use resolved display path through SearchLineStyle.path_display
• Unified path printing in execute.rs via display_path_for_candidate()
• Update StandardSink to store String display_path instead of &Path
• Fix integration tests to expect correct absolute/relative paths
• Update rg-compat-matrix.md to reflect implemented status
• No runtime rg dependency in tests; use ripgrep clone as manual reference

Behavior

• If any search scope is absolute (e.g., sift pattern /tmp/foo), paths are printed absolute
• If all scopes are relative (e.g., sift pattern ./foo or just sift pattern), paths are printed relative
• This applies to both indexed and walk search modes
• Follows ripgrep semantics for path display

Testing

• All CLI integration tests pass (90+)
• All core unit tests pass (37)
• cargo fmt --check passes
• cargo clippy -D warnings passes