feat: design validator, CSV/JSON export, 1/2A6-2 motor preset

[rramboer]

Summary

Three quick-win additions, stacked on top of #2 (the hobby-rocket pivot). Base-branch should be set to feat/hobby-rocket-pivot after #2 merges to main, or the PR can be retargeted to main.

1. Pre-launch design validator

validate_design(rocket, sim_config=None) -> list[DesignWarning] runs heuristic checks on a configured Rocket and returns advisories. The CLI runs it automatically (skip with --no-validate) and returns exit code 2 if any error-level warning fires, so it can gate CI or scripts.

Implemented checks:

Code	Severity	Trigger
motor_too_big	error	Motor diameter exceeds airframe diameter
will_not_lift	error	Peak thrust-to-weight ratio < 1 (rocket can't lift off)
lawn_dart	error	Ground impact before recovery deploys
low_twr	warning	Peak TWR between 1 and 5 (marginal launch-rod departure)
low_apogee	warning	Predicted apogee < 30 m (underpowered)
transonic	warning	Predicted max velocity > 343 m/s (airframe stress)
ballistic_descent	warning	No recovery system configured
delay_too_short	info	Ejection fires more than 2 s before apogee (chute may shred)
delay_too_long	info	Ejection fires more than 2 s after apogee (rocket descends hard)

2. CSV / JSON export

SimulationResult gains .to_csv(path), .to_dict(), and .to_json(path). CLI flags --csv FILE and --json FILE. Output formats:

• CSV: one row per simulation step, columns time_s, altitude_m, velocity_ms, acceleration_ms2, mass_kg, thrust_n, drag_n, phase.
• JSON: { "rocket_name", "summary": {...}, "states": [...] } — the summary dict mirrors the public SimulationResult fields.

3. 1/2A6-2 motor preset

The Mosquito kit was previously documented as using 1/2A6-2, but the motor was missing from MOTORS so the kit silently fell back to A8-3 via a _KIT_MOTOR_FALLBACKS table. The preset is now present and the fallback plumbing is gone. Mosquito kit's motor is now 1/2A6-2 for real.

Bonus fix

SimulationResult.deployed_below_ground previously only fired when the ejection charge tried to deploy below ground level. It now also fires when the rocket lands before recovery ever deploys (which is what validate_design's lawn_dart check needs). One-line addition in the landing branch of simulation.py.

Test plan

• pytest — 114 tests pass (was 96; +18 across test_validation.py, test_export.py, and one extra in test_motors.py)
• ruff check src tests — clean
• ruff format --check src tests — clean
• mypy src — no issues
• python -m build — produces rocket_sim-0.1.0.tar.gz and the wheel
• twine check dist/* — both pass
• CLI smoke: rocket-sim --kit alpha-iii --motor c6-5 --no-plot --csv out.csv --json out.json — runs validation, prints summary, writes both files
• CLI smoke: a custom rocket with motor diameter > airframe diameter triggers the motor_too_big error path and exits non-zero
• CI green on this PR

[rramboer]

Closing in favor of folding these commits into #2 — solo-project, no need for a stacked PR. The validator + CSV/JSON + 1/2A6-2 commit (b383d3c) is now part of #2 alongside the pivot commit.