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>

src/tilemap.c

@@ -828,14 +828,20 @@ akerr_ErrorContext *akgl_tilemap_release(akgl_Tilemap *dest)
     PREPARE_ERROR(errctx);
     FAIL_ZERO_RETURN(errctx, dest, AKERR_NULLPOINTER, "NULL map");
     int i = 0;
+    // Each pointer is cleared as it goes. Without that a second release
+    // destroys textures SDL has already freed, and the second loop used to
+    // destroy `tilesets[i]` while testing `layers[i]` -- so every tileset
+    // texture was freed twice and no image layer's texture was freed at all.
     for ( i = 0; i < AKGL_TILEMAP_MAX_TILESETS; i++ ) {
 	if ( dest->tilesets[i].texture != NULL ) {
 	    SDL_DestroyTexture(dest->tilesets[i].texture);
+	    dest->tilesets[i].texture = NULL;
 	}
     }
     for ( i = 0; i < AKGL_TILEMAP_MAX_LAYERS; i++ ) {
 	if ( dest->layers[i].texture != NULL ) {
-	    SDL_DestroyTexture(dest->tilesets[i].texture);
+	    SDL_DestroyTexture(dest->layers[i].texture);
+	    dest->layers[i].texture = NULL;
 	}
     }
     SUCCEED_RETURN(errctx);