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/game.h

@@ -177,17 +177,7 @@ extern SDL_FRect *akgl_camera;
  * @note It takes the state lock on the way in and releases it on the way out, so
  *       a failure part-way through leaves the mutex held.
  */
-akerr_ErrorContext AKERR_NOIGNORE *akgl_game_init();
-/**
- * @brief Declared but never defined. Do not call.
- *
- * There is no definition anywhere in the library, so a translation unit that
- * calls this compiles and then fails to link. Screen setup is akgl_render_2d_init.
- * Tracked in TODO.md under header/implementation surface drift.
- *
- * @return Nothing; it cannot be called.
- */
-akerr_ErrorContext AKERR_NOIGNORE *akgl_game_init_screen();
+akerr_ErrorContext AKERR_NOIGNORE *akgl_game_init(void);
 /**
  * @brief Count this frame, and recompute the frame rate once a second has passed.
  *
@@ -198,7 +188,7 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_game_init_screen();
  * the low-FPS threshold and so fires `lowfpsfunc` on every frame until the first
  * second is up.
  */
-void akgl_game_update_fps();
+void akgl_game_update_fps(void);
 /**
  * @brief Write the game state and the name-to-pointer tables to a save file.
  *
@@ -311,4 +301,51 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_game_state_unlock(void);
  */
 akerr_ErrorContext AKERR_NOIGNORE *akgl_game_update(akgl_Iterator *opflags);
 
+/*
+ * These functions are part of the internal API and should not be called by the
+ * user. They are only exposed here for unit testing.
+ */
+
+/**
+ * @brief Write all four name-to-address tables, each with its terminator.
+ *
+ * Actors, sprites, spritesheets, characters, in that order -- which is the order
+ * akgl_game_load reads them back in. Each table is the registry enumerated one
+ * entry at a time, followed by a zeroed name field and a zeroed pointer that the
+ * reader stops on.
+ *
+ * @param fp The open save-game stream. Required.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER If @p fp is `NULL`.
+ * @throws AKERR_IO On a stream error or a short write while emitting a
+ *         terminator.
+ *
+ * @note Only the terminators are written through the error-reporting path. The
+ *       entries themselves go through `SDL_EnumerateProperties`, whose callbacks
+ *       cannot report failure upward and instead terminate the process -- so a
+ *       write error mid-table never reaches this function's return value.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akgl_game_save_actors(FILE *fp);
+/**
+ * @brief Refuse a save file unless its version matches the running one exactly.
+ *
+ * Both strings are parsed as semver and compared with `"="` -- exact equality,
+ * not compatibility. That is deliberate and strict: a save file is a raw memory
+ * image of `akgl_Game` and the object tables, so any change to a struct layout
+ * invalidates it, and semver has no way to say "the layout did not move".
+ *
+ * @param versiontype What is being compared -- `"library"` or `"game"`. Used
+ *                    only to build the message, but required, since a bare
+ *                    "incompatible version" error would not say which.
+ * @param newversion  The version read out of the save file. Required.
+ * @param curversion  The version of the running program or library. Required.
+ * @return `NULL` when the two match, otherwise an error context owned by the
+ *         caller.
+ * @throws AKERR_NULLPOINTER If any of the three arguments is `NULL`.
+ * @throws AKERR_VALUE If either string is not valid semver. The message says
+ *         which side it came from.
+ * @throws AKERR_API If both parse but are not equal. The message quotes both.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akgl_game_load_versioncmp(char *versiontype, char *newversion, char *curversion);
+
 #endif //_AKGL_GAME_H_