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>

include/akgl/character.h

@@ -20,7 +20,6 @@
 #include <akgl/sprite.h>
 
 #define AKGL_CHARACTER_MAX_NAME_LENGTH  128
-#define AKGL_MAX_HEAP_CHARACTER                256
 
 /** @brief Defines reusable movement parameters and actor-state sprite bindings. */
 typedef struct akgl_Character {
@@ -62,7 +61,7 @@ typedef struct akgl_Character {
  *         #AKGL_REGISTRY_CHARACTER -- in practice, because akgl_registry_init
  *         has not run and the registry id is still 0.
  */
-akerr_ErrorContext AKERR_NOIGNORE *akgl_character_initialize(akgl_Character *basechar, char *name);
+akerr_ErrorContext AKERR_NOIGNORE *akgl_character_initialize(akgl_Character *obj, char *name);
 /**
  * @brief Bind a sprite to one exact combination of actor-state bits.
  *