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>

CMakeLists.txt

@@ -62,7 +62,7 @@ endif()
 # 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 would mean disabling both permanently to gain 5
-# -Wsign-compare. See deps/libakerror/TODO.md item 8.
+# -Wsign-compare. See libakerror issue #18.
 
 if(AKGL_COVERAGE)
   if(NOT CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")