Rule: Small Increments

Work in method-by-method steps.
- Implement small vertical slices.
- After every change: write → run → test → evaluate → fix.
- No large PRs or 1000-line commits.

When a tracker issue is active, a slice is not done until the commit and the issue update exist together. GitHub mechanics (Refs #N / Closes #N, gh issue comment) live in rule do-github-issues, not here.