Namespace every exported symbol, and bump to 0.5.0

Closes internal-consistency items 1 through 6, 12 and 13. Every include guard
is _AKGL_<FILE>_H_, every in-project header include is angled, and every
exported function, type and global carries the akgl_ prefix. This is an ABI
break; the soname goes to libakgl.so.0.5. TODO.md carries the full rename
table.

The renames were driven by renaming each declaration and letting the compiler
find the uses, not by pattern substitution: renderer, physics and camera are
also parameter and struct-member names, and a sed would have rewritten
map->physics and every akgl_RenderBackend *renderer parameter without a word.

Item 4 turned out not to be cosmetic. The library exported a global called
renderer and tests/character.c defined an SDL_Renderer *renderer of its own;
the executable's definition preempted the library's, akgl_sprite_load_json
read a SDL_Renderer * through an akgl_RenderBackend *, and every texture load
in that suite failed. The suite reported success anyway, because libakerror's
unhandled-error handler ends in exit(errctx->status), exit keeps only the low
byte, and AKGL_ERR_SDL is exactly 256. So character had been green while
running one of its four tests, and every suite in the tree was unable to fail
on the most common status in a library built on SDL.

Both are fixed. tests/testutil.h gains TEST_TRAP_UNHANDLED_ERRORS(), which
collapses any status a byte cannot carry onto 1, and every suite installs it.
character binds a real backend with akgl_render_2d_bind. Its fourth test then
runs for the first time and fails on a defect it has asserted all along, so
akgl_heap_release_character now walks state_sprites with
AKGL_ITERATOR_OP_RELEASE and destroys the property set before zeroing the slot
-- TODO.md Defects item 21 and half of Carried over item 1.

AKGL_TIME_ONESEC_MS said "one second in milliseconds" and held 1000000, so
akgl_game_state_lock waited roughly sixteen minutes rather than one second. It
is AKGL_TIME_ONEMS_NS now, the budget is its own named constant, and
tests/game.c holds the mutex from a second thread to assert the wait -- the
contended path had no coverage at all.

Headers are self-contained and it is enforced: AKGL_PUBLIC_HEADERS drives both
install() and a generated translation unit per header, so a header that ships
is a header that is checked. Writing that found registry.h, which used
SDL_PropertiesID in eight declarations and included no SDL header.

23/23 suites pass, memcheck is clean, reindent --check is clean.

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

include/akgl/tilemap.h

@@ -27,12 +27,12 @@
  *
  * Note the size of akgl_Tilemap: a layer's tile grid alone is 512x512 ints, and
  * a tileset's offset table is 65536 pairs. It is a megabytes-large object and
- * belongs in static storage, which is where `_akgl_gamemap` puts it. Do not put
+ * belongs in static storage, which is where `akgl_default_gamemap` puts it. Do not put
  * one on the stack.
  */
 
-#ifndef _TILEMAP_H_
-#define _TILEMAP_H_
+#ifndef _AKGL_TILEMAP_H_
+#define _AKGL_TILEMAP_H_
 
 #include <limits.h>
 #include <akgl/actor.h>
@@ -151,7 +151,7 @@ typedef struct {
     akgl_TilemapLayer layers[AKGL_TILEMAP_MAX_LAYERS];      /**< The layers, in draw order: index 0 is furthest back. */
 
     // Different levels may have different physics.
-    bool use_own_physics;                                   /**< Set when the map declared a `physics.model` property. A caller that honours it simulates through `physics` instead of the global backend. */
+    bool use_own_physics;                                   /**< Set when the map declared a `physics.model` property. A caller that honours it simulates through `akgl_physics` instead of the global backend. */
     akgl_PhysicsBackend physics;                            /**< This map's own backend, valid only when `use_own_physics` is set. */
 } akgl_Tilemap;
 
@@ -204,12 +204,12 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_tilemap_load(char *fname, akgl_Tilemap *
  * tile grid. An image layer ignores the viewport entirely and is drawn once at
  * the origin.
  *
- * Drawing goes through the global `renderer`, not through a backend passed in.
+ * Drawing goes through the global `akgl_renderer`, not through a backend passed in.
  *
  * @param dest     The map to draw from. Required. Not an output parameter
  *                 despite the name.
  * @param viewport The rectangle of the map, in map pixels, that is on screen.
- *                 Required. Usually the global `camera`.
+ *                 Required. Usually the global `akgl_camera`.
  * @param layeridx Which layer to draw, 0 to `numlayers - 1`. **Not**
  *                 bounds-checked: an index past the end reads a neighbouring
  *                 layer, or past the array.
@@ -490,4 +490,4 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_tilemap_release(akgl_Tilemap *dest);
  */
 akerr_ErrorContext AKERR_NOIGNORE *akgl_tilemap_scale_actor(akgl_Tilemap *map, akgl_Actor *actor);
 
-#endif //_TILEMAP_H_
+#endif //_AKGL_TILEMAP_H_