Add return-to-launch guidance

[aeharding]

Summary

• add compact return-to-launch and waypoint ETAs without adding instrument rows
• show matching magenta sunset and projected-arrival offsets inside the sunset window
• add a peripheral green turn-direction edge for stable, near-aligned returns
• hide arrival values inside one mile of launch or at one minute or less
• add replay-only RTL diagnostics, production-backed local backtesting, and estimator pressure tests

Estimator

• derive target speed from stable direct evidence when available
• otherwise estimate wind and represented airspeed from bounded heading samples
• reject wingovers, coupled throttle/vertical oscillation, stale flight levels, and unstable fits
• adapt quickly to sustained high-confidence changes without one-fix ETA cliffs

Guidance is derived from the recorded fix prefix. It adds no persisted calibration, network request, native contract, or recording lifecycle dependency.

Validation

• pnpm exec tsc --noEmit
• pnpm exec eslint . --max-warnings 0
• pnpm format:check
• pnpm check:css
• pnpm test (529 passed)
• pnpm exec playwright test (106 passed, 10 skipped)
• git diff --check
• independent architecture and adversarial estimator audits: no blockers

The bounded desktop benchmark measured about 0.14% of one core at a 1 Hz fix rate. Physical iPhone battery and thermal measurement remains a follow-up drill.