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>

docs/23-appendix-limits.md

@@ -301,7 +301,8 @@ A ninth child is `AKERR_OUTOFBOUNDS`; an over-long name is silently truncated.
 
 `akgl_sprite_load_json` bounds the `frames` array against `AKGL_SPRITE_MAX_FRAMES` before
 writing anything, and refuses a frame number that will not fit a `uint8_t` rather than
-truncating it into an index naming a different tile (`TODO.md` item 16).
+truncating it into an index naming a different tile (`TODO.md`, "Found while rewriting
+the Doxygen comments").
 
 **`AKGL_SPRITE_MAX_REGISTRY_SIZE` is dead.** It is defined in `sprite.h` and referenced
 nowhere in `src/`, `include/`, `tests/` or `util/`. It bounds nothing. Do not size anything
@@ -335,8 +336,8 @@ Three corrections to what those comments say, all verified against `src/tilemap.
 - **`AKGL_TILEMAP_MAX_OBJECTS_PER_LAYER` *is* enforced.** The comment above says it is not.
   `akgl_tilemap_load_layer_objects` bounds `j` at the top of the loop body and raises
   `AKERR_OUTOFBOUNDS`; `akgl_tilemap_load_tilesets` does the same for
-  `AKGL_TILEMAP_MAX_TILESETS`. Both landed in 0.5.0 (`TODO.md`, "Known and still open"
-  item 17), and the header comment was not updated with them.
+  `AKGL_TILEMAP_MAX_TILESETS`. Both landed in 0.5.0 (`TODO.md`, "Found while rewriting
+  the Doxygen comments"), and the header comment was not updated with them -- issue #63.
 - **Width and height are not bounded individually.** `akgl_tilemap_load` checks
   `width * height >= AKGL_TILEMAP_MAX_WIDTH * AKGL_TILEMAP_MAX_HEIGHT`, and the comparison
   is `>=`, so the true ceiling is **262143 tiles** in any shape. A 1024×256 map loads; a