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>

tests/json_helpers.c

@@ -66,7 +66,7 @@ static akerr_ErrorContext *load_fixture(void)
  * usually because something is not releasing -- arrived as a segfault inside
  * strncpy.
  *
- * This is what TODO.md Performance item 29 looked like from the outside: not
+ * This is what the tilemap string-pool leak looked like from the outside: not
  * "the tilemap loader leaks five strings a load", but a crash somewhere else
  * entirely, fifty levels later.
  */

tests/perf_render.c

@@ -483,7 +483,7 @@ static akerr_ErrorContext *bench_asset_load(void)
 	    // The 52nd load would find the pool empty, and what happens then is
 	    // not an AKGL_ERR_HEAP: akgl_get_json_string_value finishes with
 	    // pass-up false, swallows the failed claim, and dereferences the
-	    // pointer it never set. Both defects are filed in TODO.md; this loop
+	    // pointer it never set. Both defects are recorded in TODO.md; this loop
 	    // works around the first so it never reaches the second.
 	    for ( j = 0; j < AKGL_MAX_HEAP_STRING; j++ ) {
 		akgl_heap_strings[j].refcount = 0;

tests/physics_sim.c

@@ -31,7 +31,7 @@
  *       inside simulate produced a longer step and a red suite. It failed
  *       exactly once, under a parallel ctest, which is the worst way to find
  *       out. That the engine cannot be stepped exactly is itself a finding;
- *       see TODO.md.
+ *       see issues #29 through #32.
  */
 
 #include <SDL3/SDL.h>

tests/renderer.c

@@ -10,7 +10,7 @@
  *
  * akgl_render_2d_init() is not covered here: it creates a window from the
  * property registry and writes the `camera` global, which is what the offscreen
- * harness described in TODO.md exists to make testable.
+ * harness described in issue #6 exists to make testable.
  */
 
 #include <SDL3/SDL.h>