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>

include/akgl/registry.h

@@ -23,7 +23,7 @@
  *          no-op akgl_set_property and an akgl_get_property that always hands
  *          back the caller's default, which in turn means akgl_render_2d_init
  *          and akgl_physics_init_arcade quietly ignore their configuration.
- *          TODO.md, "Known and still open" item 3.
+ *          TODO.md, "Formatting and hygiene".
  */
 
 #ifndef _AKGL_REGISTRY_H_
@@ -170,7 +170,7 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_registry_load_properties(char *fname);
  * @note The name table it reads disagrees with actor.h in two places: bits 11
  *       and 12 are named `UNDEFINED_11`/`UNDEFINED_12` rather than `MOVING_IN`
  *       and `MOVING_OUT`, so those two states cannot be named from JSON at all.
- *       TODO.md items 24-26.
+ *       TODO.md, "AKGL_ACTOR_STATE_STRING_NAMES".
  */
 akerr_ErrorContext AKERR_NOIGNORE *akgl_registry_init_actor_state_strings(void);
 /**