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/12-actors.md

@@ -331,7 +331,7 @@ backend accumulates gravity, so tapping down mid-jump stopped the character in t
 air. Velocity was never theirs to clear either — `simulate` recomputes `v` as
 `e + t` every step. Those notes predate the fix.
 
-> **Known defect (`TODO.md`, "Arcade physics feel").** There is no friction and no
+> **Known defect (issue #30).** There is no friction and no
 > deceleration: zeroing `tx` on release **stops the actor dead**. Correct for Zelda,
 > wrong for Mario. Bind your own `_off` handler that decays `tx` over time if you
 > want momentum.
@@ -431,12 +431,12 @@ for `akgl_Actor`.
   `akgl_actor_initialize` writes the registry entry under the *caller's* `name`
   pointer while `akgl_heap_release_actor` clears it under the actor's truncated
   128-byte copy. Identical for ordinary names; divergent past 127 bytes, and then
-  the registry keeps an entry pointing at a zeroed slot. Related to `TODO.md`,
-  "Truncated registry keys can collide".
+  the registry keeps an entry pointing at a zeroed slot. That is issue #38;
+  issue #54 covers the related truncation collision.
 - **`akgl_actor_initialize` silently replaces a same-named actor**, and the one it
   displaced becomes unreachable rather than being released.
 - **No friction on release**, and **no terminal velocity** under gravity —
-  `TODO.md`, "Arcade physics feel". [Chapter 14](14-physics.md) covers both.
+  issues #29 through #32. [Chapter 14](14-physics.md) covers both.
 
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