Auto-resume flight across chunk unload/reload

When the ship's chunk unloads the CC computer powers off and its redstone
outputs reset to 0, so on return the ship dropped and the pilot had to
re-ENGAGE every time. Flight state now survives the reboot:

- engaged.txt flag written while flying, cleared on pilot STOP.
- On boot, if engaged + profile present: drive the hover baseline
  immediately (closes the fall gap), then runStandby(true) re-enters the
  flight loop automatically with the persisted target -- no manual ENGAGE.
- runFly drives hover before its first sleep so no entry path leaves a
  0-output gap.
- Cleanup no longer lifts a PARKED ship: Ctrl+T out of standby holds hover
  only when engaged, else valves off.
- engaged.txt registered as per-ship state (gitignore, installer keep/reset,
  menu reset, build excludes).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -63,7 +63,26 @@ Boot chain (autopilot): `startup.lua` → `ap boot` → `autopilot/main.lua`.
 Menu any time: `Ctrl+T`, then `ap`. (`stabilizeV2` remains as an alias.)
 
 Ship state lives on the computer only: `sides.txt`, `calib.txt`, `target.txt`,
-`settings.txt`. Reset it via menu `p` → `R`, or when re-running the installer.
+`settings.txt`, `engaged.txt`. Reset it via menu `p` → `R`, or when re-running
+the installer.
+
+## Surviving chunk unload / reload
+
+When you fly away and the ship's chunk unloads, the CC computer powers off and
+its redstone outputs reset to 0 — so on its own the ship would drop the moment
+you return. The autopilot handles this:
+
+- While flying it keeps an `engaged.txt` flag. On boot (`startup.lua` runs
+  automatically when the chunk reloads) it drives the hover baseline
+  **immediately**, then re-enters the flight loop on its own — no manual
+  ENGAGE, holding the same target it had.
+- A pilot **STOP** clears the flag, so a ship you deliberately stopped stays
+  stopped across reloads. **HOLD**/**LAND** keep flying, so they resume.
+
+There is still an unavoidable ~1–2 s gap while the computer boots; on this slow
+ship that is harmless. If the computer comes back *off* after a reload (some
+server setups), right-click it once — CC then runs `startup.lua` and the
+auto-resume takes over.
 
 ## Updating the code