Phase 19 — Exercises: Capstone — Maintainer & Startup

Work these after the labs. They escalate from "explain it" to "design it" — staff-level means you can do the last ones cold.

1. Write a merge-ready PR description for a small fix (scope, tests run, why not a duplicate).
2. Pick a model from this course and propose, with numbers, the single highest-ROI optimization.
3. Draft a one-paragraph startup thesis with a defensible moat per CAREER.md Track C.

Self-grading

For each: could you (a) explain it to a teammate in 2 minutes, and (b) point to the exact upstream/ file that proves your answer? If not, re-read the matching anchor in 01-deep-dive.md.