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>

README.md

@@ -184,7 +184,7 @@ AKGL_BENCH_SCALE=0.1 ctest --test-dir build -L perf  # a tenth of the iterations
 
 Each measurement is the best of five runs and is held to a budget set at roughly ten times the recorded baseline, so a suite that turns red means an algorithmic regression rather than a busy machine. Budgets are enforced only in an optimized build at full scale; below `AKGL_BENCH_SCALE=1.0`, and in a coverage build, they are reported without failing.
 
-`PERFORMANCE.md` carries the recorded baseline, the frame budget it adds up to, and what the numbers say — including the raw-SDL control rows that separate what libakgl costs from what the rasterizer costs. The six defects the stress tests turned up, and the targets the numbers are measured against, are in `TODO.md` under **Performance**.
+`PERFORMANCE.md` carries the recorded baseline, the frame budget it adds up to, and what the numbers say — including the raw-SDL control rows that separate what libakgl costs from what the rasterizer costs. The six defects the stress tests turned up, and the targets the numbers are measured against, are in `TODO.md` under **Performance**; the plan for the missed targets is issue #60.
 
 ## Memory checking
 
@@ -211,7 +211,7 @@ this tree, that notice has to travel with it.
 It is the only dependency in that position. SDL3, SDL3_image, SDL3_mixer, SDL3_ttf, jansson,
 libakerror and libakstdlib are all linked as separate shared objects and carry their own
 notices; `deps/semver` is a single header, MIT, installed alongside ours; and `deps/tg` is
-vendored but **compiled into nothing** -- see `TODO.md`, "tg is vendored and has no
+vendored but **compiled into nothing** -- see `TODO.md`, "Why deps/tg is still in the
 consumer".
 
 libccd is given a static arena rather than the allocator it ships with, so "libakgl does not