Report the failures that used to be crashes

Closes Defects items 30 and 31 and Known-and-still-open items 1, 2, 5, 9 and 11.

Both string accessors in json_helpers.c ended their ATTEMPT block with
FINISH(errctx, false), which swallows the failure, and then strncpy'd through
the pointer akgl_heap_next_string never set. So the one condition the pool
exists to report -- it is full, which in practice means something is not
releasing -- arrived as a segfault somewhere else entirely. It is
FINISH(errctx, true) now, and tests/json_helpers.c claims every slot and
asserts AKGL_ERR_HEAP comes back out of both. That test segfaults against the
old code, which is also how the tilemap leak test in the previous commit
confirmed this one.

akgl_tilemap_release tested layers[i].texture and destroyed
tilesets[i].texture, so every tileset texture was freed twice on one release
and no image layer's texture was freed at all. Pointers are cleared as they go,
so a second release is safe instead of a use-after-free.

akgl_game_update_fps called game.lowfpsfunc() unguarded, on a path taken on
frame one because fps is 0 for the first second. Only akgl_game_init installs
it, and renderer.h documents the other path deliberately -- a host that owns
its window binds a backend instead. It installs the default when it finds NULL.

akgl_controller_pushmap and akgl_controller_default checked only the upper
bound, so a negative id indexed before akgl_controlmaps.

The two test-harness helpers were quietly worthless. akgl_render_and_compare
drew t1 on both passes, so it always reported a match and every image assertion
built on it asserted nothing; and akgl_compare_sdl_surfaces memcmp'd
s1->pitch * s1->h bytes out of both surfaces without checking that the second
was the same size, so a smaller one was read past its end. Both fixed, both
tested. tests/util.c also now calls the collide-point test it has defined and
never run.

25/25 pass, memcheck clean, reindent --check and check_error_protocol clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

include/akgl/game.h

@@ -92,7 +92,7 @@ typedef struct {
     SDL_Time lastIterTime;      /**< Timestamp of the most recent akgl_game_update_fps call. */
     SDL_Time lastFPSTime;       /**< When `fps` was last recomputed. */
     int16_t framesSinceUpdate;  /**< Frames counted so far in the current second. */
-    void (*lowfpsfunc)(void);   /**< Called every frame while `fps` is under 30. Defaults to akgl_game_lowfps; replace it to do something more useful than log. */
+    void (*lowfpsfunc)(void);   /**< Called every frame while `fps` is under 30. akgl_game_init installs akgl_game_lowfps, and akgl_game_update_fps installs it too if it finds this NULL -- an embedder that binds its own renderer rather than calling akgl_game_init used to crash here on frame one. Replace it to do something more useful than log. */
 } akgl_Game;
 
 /** @brief The SDL window, created by akgl_render_2d_init. `NULL` until then. */