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/06-the-registry.md

@@ -35,8 +35,8 @@ same `SDL_Texture`, and the texture is loaded once.
 entry `i` to `1 << i`, which is what lets a character definition write
 `"AKGL_ACTOR_STATE_FACE_LEFT"` instead of `2`. Two entries in that name table disagree with
 `actor.h`: bits 11 and 12 are `UNDEFINED_11` and `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). The individual name registrations are not checked, either.
+`MOVING_OUT`, **so those two states cannot be named from JSON at all** (`TODO.md`,
+"`AKGL_ACTOR_STATE_STRING_NAMES`"). The individual name registrations are not checked, either.
 
 **The music registry is empty.** `akgl_registry_init_music` creates it and nothing in the
 library ever writes to it. It is there for you.
@@ -51,8 +51,7 @@ around.
 Read against `src/registry.c` and `src/game.c`, this is what is true:
 
 - `akgl_registry_init` creates **eight**, including `AKGL_REGISTRY_PROPERTIES`. The
-  properties call was added in 0.5.0 (`TODO.md`, "Known and still open" item 3, marked
-  fixed). Its order is spritesheet, sprite, character, actor, actor-state-strings, font,
+  properties call was added in 0.5.0 (`TODO.md`, "Formatting and hygiene"). Its order is spritesheet, sprite, character, actor, actor-state-strings, font,
   music, properties.
 - **`akgl_game_init` never calls `akgl_registry_init`.** It calls the eight individual
   initializers itself, in a different order: actor, sprite, spritesheet, character, font,
@@ -113,12 +112,11 @@ document. The fields:
 
 **Two distinct names that agree on their first 127 bytes truncate to the same key**, and
 the second `SDL_SetPointerProperty` silently replaces the first. The objects are different;
-the registry cannot tell. That is recorded in `TODO.md`, "Truncated registry keys can
-collide", and it is not being fixed because the fix is a contract change — refusing an
+the registry cannot tell. That is issue #54, and it is not fixed yet because the fix is a contract change — refusing an
 over-long name with `AKERR_OUTOFBOUNDS` — and every one of those headers currently promises
 the opposite.
 
-There is a second half that the `TODO.md` entry understates. It says the truncated name *is*
+There is a second half that the issue understates. It says the truncated name *is*
 the registry key. That is true for sprites and spritesheets, which register under their own
 copied field. It is **not** true for actors and characters: