Fix watch mode with absolute input paths

[Companion]

Closes #309

Problem

Watch mode silently ignores file changes when the configured input is an absolute path inside the current working directory.

File watcher events use absolute paths, while the worker tree stores paths using the same representation as the configured input. The watcher previously made event paths relative to the current directory unconditionally, causing them to no longer match the absolute worker tree paths.

The initial build still succeeded, making the watcher appear operational despite subsequent changes being ignored.

Change

Only make watcher event paths relative when the configured input path is also relative.

The mapping remains lexical, allowing removed files to be handled without requiring them to exist. Existing symlink remapping continues to take precedence.

Testing

Added focused tests covering:

• Relative and absolute input paths
• Events inside and outside the current directory
• Missing current directory information
• Symlink remapping precedence
• Removed or nonexistent paths

Also verified watch mode using relative and absolute inputs with automatically discovered, explicitly relative and explicitly absolute configuration paths. Changes to primary files, external dependencies and configuration files were detected in every configuration.

Verified with:

• cargo test -q
• cargo fmt --check
• cargo clippy --lib --bins --tests --all-features -- -D warnings
• lua ./scripts/test-commands.lua
• git diff --check