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>

docs/18-audio.md

@@ -302,7 +302,7 @@ touches `akgl_mixer`, so calling only that leaves `akgl_load_start_bgm` handing
 `src/assets.c:43` sets `MIX_PROP_PLAY_LOOPS_NUMBER` on it, and `src/assets.c:45` plays the
 track with it. **0 is SDL's "no property set" sentinel**, not a set this function owns, so the
 write is rejected — unchecked — and the play call is given no options. The music plays once
-and stops, and `akgl_load_start_bgm` reports success either way. `TODO.md` item 19; the fix
+and stops, and `akgl_load_start_bgm` reports success either way. Issue #16; the fix
 is `SDL_CreateProperties()` into `bgmprops`, checked, and destroyed in `CLEANUP`.
 
 **`AKGL_REGISTRY_MUSIC` exists and nothing populates it.** `akgl_registry_init_music()`