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/10-spritesheets-and-sprites.md

@@ -295,14 +295,14 @@ game that wants one binds its own `changeframefunc`.
 - **Truncated names are registry keys.** `akgl_Sprite::name` is 128 bytes and
   `akgl_SpriteSheet::name` is 512. A longer name truncates *silently*, and two names
   that truncate the same collide — the second registration replaces the first with no
-  error. Recorded in `TODO.md`, "Truncated registry keys can collide"; the fix is a
+  error. Recorded as issue #54; the fix is a
   contract change, since the headers currently promise truncation.
 - **The heap acquire functions are asymmetric.** `akgl_heap_next_string` increments
-  `refcount`; `next_sprite` and `next_spritesheet` do not. `TODO.md` item 8. See
+  `refcount`; `next_sprite` and `next_spritesheet` do not. Issue #14. See
   [Chapter 05](05-the-heap.md).
 - **No test asserts a clean sprite or spritesheet load/release cycle.** The tilemap
   cycle is asserted over 64 iterations; the sprite, spritesheet and character ones
-  are not. `TODO.md`, "Targets", row 16.
+  are not. `TODO.md`, "Targets", row 16; issue #13.
 - **`akgl_spritesheet_coords_for_frame` bounds-checks nothing.** See above.
 
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