Give every exported function a declaration, and check that it stays that way

Closes internal-consistency items 7 through 15. Nineteen non-static functions
were in the ABI with no declaration anywhere, so no consumer could call them
and any consumer could collide with them.

The four gamepad_handle_* functions are the ones that mattered: controller.h
declared akgl_controller_handle_button_down and three siblings that did not
exist, so anything compiled against the header alone failed to link. The
definitions carry the declared names now, which also closes Defects -> Known
and still open item 10, and their documentation moved to the header.

The rest are either declared under a "part of the internal API" block --
akgl_game_save_actors and akgl_game_load_versioncmp, which tests/game.c had to
declare for itself, plus six tilemap loader helpers the untested-loader work
wants to reach -- or static, which is what the four save iterators and
load_objectnamemap should always have been. akgl_path_relative_from is deleted:
declared nowhere, called from nowhere, never wrote its output, and leaked a
pooled string on every call, so it closes Known and still open item 4 and item
40 by ceasing to exist.

scripts/check_api_surface.sh keeps it closed. It reads the built library's
dynamic symbol table and every public header with comments stripped, and fails
on an exported akgl_* symbol that is declared nowhere. Stripping comments is
the whole point -- four of these were mentioned in controller.h prose, which is
how they went unnoticed.

The pool-size ceilings are defined once, in heap.h, so the #ifndef override
hook fires for the first time; actor.h, sprite.h and character.h were defining
the same four unconditionally from headers heap.h includes above its own guard.
tests/header_pool_override.c fails the compile if that regresses.

Also here: (void) rather than () on the twelve no-argument entry points,
AKERR_NOIGNORE only on declarations, static helpers with the akgl_ prefix
dropped, and the six parameter-name mismatches. akgl_get_json_with_default had
its two contexts swapped rather than merely misspelled -- the incoming one was
`err` and its own was `e`, which is the name reserved for an incoming one.

24/24 pass, reindent --check clean.

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

src/sprite.c

@@ -67,7 +67,7 @@ akerr_ErrorContext *akgl_spritesheet_coords_for_frame(akgl_Sprite *self, SDL_FRe
  * @throws AKGL_ERR_SDL If the image cannot be decoded.
  * @throws AKGL_ERR_HEAP If the spritesheet or string pool is exhausted.
  */
-static akerr_ErrorContext *akgl_sprite_load_json_spritesheet(json_t *json, akgl_SpriteSheet **sheet, char *relative_path)
+static akerr_ErrorContext *sprite_load_json_spritesheet(json_t *json, akgl_SpriteSheet **sheet, char *relative_path)
 {
     PREPARE_ERROR(errctx);
     json_t *spritesheet_json = NULL;
@@ -145,7 +145,7 @@ akerr_ErrorContext *akgl_sprite_load_json(char *filename)
 	    "Error while loading sprite from %s on line %d: %s", filename, error.line, error.text
 	    );
 
-	CATCH(errctx, akgl_sprite_load_json_spritesheet((json_t *)json, &sheet, dirname(filename_copy->data)));
+	CATCH(errctx, sprite_load_json_spritesheet((json_t *)json, &sheet, dirname(filename_copy->data)));
 	CATCH(errctx, akgl_get_json_string_value((json_t *)json, "name", &spritename));
 	CATCH(errctx,
 	      akgl_sprite_initialize(