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>

AGENTS.md

@@ -309,7 +309,7 @@ and are unaffected either way.
 **Never silence a warning with a cast.** Three `-Wpointer-sign` findings in
 `src/sprite.c` were real signedness mismatches -- `uint32_t *` passed where an
 `int *` was expected -- and a cast would have hidden every one. That is the
-whole argument of `TODO.md` item 37. Read into a correctly typed local, check
+whole argument of issue #5. Read into a correctly typed local, check
 the range, and assign.
 
 ### No repository-wide formatter
@@ -496,8 +496,8 @@ to that device are accepted and the `ENOSPC` surfaces at the flush.
 
 **Do not convert the pure-arithmetic calls.** `memset`, `memcpy`, `strlen`,
 `strcmp` and friends can only fail on a NULL argument, and the tree is
-inconsistent about them already; see `TODO.md`, "libakstdlib wrappers not yet
-adopted", before starting a sweep.
+inconsistent about them already; see issue #19, which is the decision this
+waits on, before starting a sweep.
 
 **Never silence `-Wformat-truncation`.** `include/akgl/error.h` still exports
 `DISABLE_GCC_WARNING_FORMAT_TRUNCATION` and nothing uses it. Both sites that