Patch the flash checksum into the composed Arrow delete tunes

The composed .hex files were missing a valid flash checksum entirely
(checksum.py flagged MISMATCH on both) -- not flash-ready as committed.
Patches the checksum in-place for both, and fixes compose_arrow_delete.py
to patch it automatically on future regenerations instead of leaving it
as a manual follow-up step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FP2GaxS9HkUdL5sLBnjKje

tunie/research/reference-maps/compose_arrow_delete.py

@@ -38,6 +38,7 @@ from __future__ import annotations
 
 import sys
 
+import checksum
 from decode_map import decode, encode
 from toggle_devices import SAI, O2_1, O2_2, repack, unpack
 
@@ -77,6 +78,11 @@ def compose(arrow_raw: bytes, stock_after_raw: bytes, delete_raw: bytes) -> tupl
             f"  (byte {before} -> {after})"
         )
 
+    before_sum = checksum.stored(bytes(arrow_rom))
+    checksum.patch(arrow_rom)
+    after_sum = checksum.compute(bytes(arrow_rom))
+    notes.append(f"checksum: stored {before_sum:04x} (stale) -> patched {after_sum:04x}")
+
     repacked = repack(arrow_dec, bytes(arrow_rom), positions)
     out = encode(repacked)
     assert decode(out) == repacked, "round-trip failed"
@@ -101,8 +107,9 @@ def main() -> int:
     for n in notes:
         print(" ", n)
     print(f"wrote {out_path} ({len(out)} bytes)")
-    print("NOTE: flash checksum not patched -- not known to be flashable as-is.")
     print("NOTE: main VE fuel-table overlap (~1340 bytes) still unresolved -- see COMMUNITY_TUNING.md.")
+    print("NOTE: only device flags + 2 idle-CO trim bytes are composed -- the main")
+    print("      running-range VE fuel tables are untouched stock-Arrow calibration.")
     return 0