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/21-tutorial-jrpg.md

@@ -536,7 +536,7 @@ state is skipped rather than reported, so every NPC in the town disappears on fr
 silently, and so does the player as soon as they stop walking.
 
 Clearing the field leaves the facing bits wherever they were last set, which is what a
-top-down game wants. `TODO.md` tracks making the default behave; until then this loop belongs
+top-down game wants. Issue #39 tracks making the default behave; until then this loop belongs
 in every game with a standing NPC in it.
 
 The loop covers the player as well as the NPCs, which is what you want — a player who stops
@@ -748,7 +748,7 @@ The physics step writes a child's `x` and `y` as an absolute world position, and
 `akgl_actor_render` adds the parent's position to it a second time. Detaching takes the
 branch that does not add it, and `CLEANUP` puts the parent back on every path including the
 failing one — an actor left with a `NULL` parent would be simulated as a free agent on the
-next step. This is `TODO.md`, "Known and still open"; when the two agree on one reading, this
+next step. This is issue #37; when the two agree on one reading, this
 hook becomes `akgl_actor_render` again.
 
 Run now. A second character walks a fixed distance behind and below the player.
@@ -892,7 +892,7 @@ Guard against the empty string:
 ```
 
 `akgl_text_rendertextat` refuses a zero-width string while `akgl_text_measure` accepts one.
-The disagreement is recorded in `TODO.md` under "Known and still open"; the check above is
+The disagreement is issue #37; the check above is
 the guard until it is settled.
 
 ### Drawing it over the world
@@ -1132,8 +1132,8 @@ than the `NULL` chapter 20 passes. `AKGL_ITERATOR_OP_UPDATE` asks for the update
 registry and `SDL_Quit` destroys it, taking the last reference to every font with no way left
 to close them.
 
-`akgl_tilemap_release` is not called. It double-frees tileset textures, which is `TODO.md`
-"Known and still open" item 2; `SDL_Quit` reclaims them correctly. A game that loads a second
+`akgl_tilemap_release` is not called. It double-freed tileset textures, which is `TODO.md`
+"Defects"; `SDL_Quit` reclaims them correctly. A game that loads a second
 level has to unwind properly, and that is what the fix will make possible.
 
 The pools are static storage and the process is exiting, so there is nothing else to free.