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>

AGENTS.md

@@ -297,8 +297,8 @@ backend vtable entry or an SDL callback must match a signature whether it reads
 every argument or not, and 4 `-Wimplicit-fallthrough` from libakerror's own
 `PROCESS`/`HANDLE`/`HANDLE_GROUP`, which fall through between `case` labels by
 design. Adopting it today would mean disabling both permanently to gain 5
-`-Wsign-compare`. The fallthrough half is filed upstream as
-`deps/libakerror/TODO.md` item 8; revisit when that lands.
+`-Wsign-compare`. The fallthrough half is filed upstream as `libakerror` issue #18;
+revisit when that lands.
 
 **Vendored code is exempt, not fixed.** `deps/semver/semver.c` is listed
 directly in `add_library(akgl ...)`, so the target's `PRIVATE` options reach it;