Make watch mode rerun only affected tests #11

Motivation

Watch mode currently reruns the discovered test set after changes. As projects grow, rerunning unaffected tests makes the feedback loop slow.

Proposed work

• Track source and test file changes between runs.
• Immediately rerun a changed test file.
• Add a dependency strategy for mapping changed source files to affected tests.
• Fall back safely to the full suite when dependency information is unavailable.
• Preserve debounce behavior and expose why each test was selected.

Acceptance criteria

• Editing one test file reruns only that file by default.
• Source changes use documented affected-test behavior.
• Deletes, renames, and newly added tests are handled.
• A full-rerun command remains available in watch mode.