Make watch mode rerun affected tests

[glennmichael123]

Closes #11

Summary

• replace the inert watch stub with portable source and test snapshots
• rerun changed or newly added tests directly and map source edits through transitive relative imports
• fall back to the full suite for deletions, renames, unresolved module imports, and unmapped source changes
• retain debounce behavior, print every selection reason, and add the interactive r full-rerun command
• document the affected-test strategy and safe fallback rules

Verification

• zig fmt --check .
• zig build test
• zig build examples
• zig build -Dtarget=x86_64-windows