Fix watcher cleanup, empty-reload guard, and stale README

[jpr5]

Summary

Post-merge follow-up to PR #40. Addresses 3 findings from code review:

• Watcher error handler now clears debounce timer and closes FSWatcher to avoid indeterminate state after fs.watch errors
• Reload guards against replacing fixtures with empty array on parse failure, preserving previous working fixtures
• Update README Future Direction to reflect that --validate-on-load and validateFixtures() now exist

Test plan

• All 565 existing tests pass
• Manually verify watcher error recovery (kill the watched file's filesystem)
• Verify reload with invalid JSON preserves previous fixtures

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/CopilotKit/llmock/@copilotkit/llmock@43

commit: b6d756f