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>

include/akgl/collision.h

@@ -74,7 +74,7 @@ struct akgl_CollisionWorld;
  *
  * Sub-stepping bounds how far an actor moves between collision tests, which
  * stops anything moving at ordinary speeds from passing through a wall. A
- * projectile is not moving at ordinary speeds. See `TODO.md`.
+ * projectile is not moving at ordinary speeds. See issue #56.
  */
 #define AKGL_COLLISION_FLAG_BULLET      0x00000008u
 
@@ -557,8 +557,8 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_sync_actors(akgl_CollisionWorl
  * Follows the convention every pool here uses: akgl_heap_next_collision_proxy
  * finds a free slot and does **not** claim it, and this takes the reference.
  * Write the two adjacent, because until the reference is taken the slot is still
- * free and the next acquire hands out the same pointer. That is `TODO.md` "Known
- * and still open" item 8, it applies to four of the five existing pools, and
+ * free and the next acquire hands out the same pointer. That asymmetry is issue
+ * #14, it applies to four of the five existing pools, and
  * this one does not depart from it -- a sixth convention would be worse than the
  * defect.
  *