Repoint every stale TODO.md citation in the tree

A sweep of all 265 tracked files, not just the ones the migration touched.

Two cross-repository citations named deps/libakerror/TODO.md item 8, a
numbering that has not existed for two releases; both now name libakerror
issue #18. Three tutorial and plan references pointed at TODO.md for defects
that are tracked: the invisible-on-frame-one actor is #39, the control map
traps are #81, and the pkg-config gap is #17.

plan.md is a live instruction document for writing the manual and still told
its reader to file findings in TODO.md; it says the tracker now.

Filed while sweeping: a zeroed akgl_ControlMap matches gamepad South, because
SDL_GAMEPAD_BUTTON_SOUTH is 0 and the two arms of the match are one
expression, and handler_on/handler_off are called without a NULL check. Both
are documented as workarounds in chapter 21 and neither had an issue. #81.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

docs/20-tutorial-sidescroller.md

@@ -1064,8 +1064,7 @@ Add one line where you set up the player:
 The default facing logic clears every facing bit and sets one from the *movement* bits — so
 an actor that stops moving is left facing nowhere. Its state drops to bare `ALIVE`, which
 your character has no sprite for, and it stops being drawn. Clearing this field leaves the
-facing bits wherever the control handlers put them. Making the default behave is tracked in
-`TODO.md`.
+facing bits wherever the control handlers put them. Making the default behave is issue #39.
 
 Run now. The arrow keys walk the player, the run animation plays, and the player stops at
 walls.