Move outstanding work from TODO.md into the issue tracker

TODO.md carried two records in one file: what had been done, with the
measurements behind it, and what was left. The second half is what a tracker
is for, and keeping it here has already cost something -- AGENTS.md records a
round where eleven entries described code that had already changed, and this
file admitted to three more.

Every open item is now an issue on source.starfort.tech/andrew/libakgl,
labelled by kind and blast radius and milestoned by what it can land in: 0.9.x
for anything that breaks no ABI, 0.10.0 for new or changed public symbols,
1.0.0 for the design work. Four are epics: the performance plan (#60),
coverage (#61), actor rotation (#62), and the false header comments (#63).

Verified against the tree before filing rather than transcribed. Three entries
were already fixed and were not filed: the akgl_path_relative context leak, the
akgl_draw_background test extension, and the SDL enumeration audit -- keyboards,
gamepads and mappings are all freed in CLEANUP today. Two were reworded because
the code had moved: the fonts item is a missing teardown entry point rather than
a missing API, since akgl_text_unloadallfonts exists, and draw_world's tilemap
call is already bounded by numlayers, so only the per-layer actor rescan remains.

TODO.md keeps the part a tracker has no place for: why a decision went the way
it did, what the measurement was, and which arguments turned out to be wrong.

TODO.txt is deleted. Four of its eight entries had shipped -- actor-to-actor
collision, actor-to-world collision, automatic facing, image layers -- and the
four that had not are #74 through #77, with the GPU renderer's research links
kept because that is the part that took the time.

Every reference that named an item number or a moved section is repointed, in
the manual, the headers, the tests and the examples.

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

examples/jrpg/jrpg.c

@@ -347,8 +347,7 @@ static akerr_ErrorContext *frame(long frameno)
  * - akgl_tilemap_release is **not** called. Its layer loop destroys
  *   `tilesets[i].texture` rather than `layers[i].texture`, so it double-frees
  *   every tileset texture and never frees an image layer's, and it NULLs
- *   nothing, so a second call is a use-after-free. TODO.md, "Known and still
- *   open" item 2. `SDL_Quit` reclaims the textures correctly; calling the
+ *   nothing, so a second call is a use-after-free. TODO.md, "Defects". `SDL_Quit` reclaims the textures correctly; calling the
  *   function that is supposed to would be worse than not.
  * - The pools are static storage. There is nothing to free and the process is
  *   about to exit.

examples/jrpg/textbox.c

@@ -83,8 +83,8 @@ akerr_ErrorContext *jrpg_textbox_draw(void)
     // akgl_text_rendertextat refuses the empty string -- SDL_ttf reports "Text
     // has zero width" and the library passes that on as AKERR_NULLPOINTER,
     // while akgl_text_measure accepts it. The two disagree, so anything drawing
-    // a line that might be empty checks for it. TODO.md, "Known and still
-    // open".
+    // a line that might be empty checks for it. TODO.md, "Found while
+    // rewriting the Doxygen comments".
     if ( textbox_text[0] == '\0' ) {
 	SUCCEED_RETURN(errctx);
     }

examples/sidescroller/player.c

@@ -10,7 +10,7 @@
  * `movementlogicfunc`, which is the only hook the step calls.
  *
  * Two of the library's documented gaps are worked around here rather than
- * papered over. Both are in `TODO.md` under "Arcade physics feel".
+ * papered over. Both are issues #29 through #32.
  */
 
 #include <math.h>
@@ -324,7 +324,7 @@ akerr_ErrorContext *ss_player_controls(int controlmapid, char *actorname)
     FAIL_ZERO_RETURN(errctx, actorname, AKERR_NULLPOINTER, "actorname");
     /* akgl_controller_pushmap checks the upper bound and not the lower one, so
      * a negative id indexes before the start of akgl_controlmaps. TODO.md,
-     * "Known and still open" item 11. */
+     * "Defects". */
     FAIL_NONZERO_RETURN(
 	errctx,
 	((controlmapid < 0) || (controlmapid >= AKGL_MAX_CONTROL_MAPS)),