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/text.h

@@ -13,8 +13,8 @@
  * static text redrawn every frame.
  */
 
-#ifndef _TEXT_H_
-#define _TEXT_H_
+#ifndef _AKGL_TEXT_H_
+#define _AKGL_TEXT_H_
 
 #include <SDL3/SDL.h>
 #include <SDL3_ttf/SDL_ttf.h>
@@ -73,7 +73,7 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_text_unloadfont(char *name);
  * @brief Rasterize a string and blit it at a screen position, in one call.
  *
  * Renders blended (anti-aliased, alpha-blended) through SDL_ttf, uploads the
- * result to a texture, draws it through the global `renderer`, and destroys both
+ * result to a texture, draws it through the global `akgl_renderer`, and destroys both
  * the texture and the surface before returning. The text is drawn at its natural
  * size -- @p x and @p y are the top-left corner, not a centre.
  *
@@ -91,9 +91,9 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_text_unloadfont(char *name);
  * @param y          Top edge of the text, in screen pixels.
  * @return `NULL` on success, otherwise an error context owned by the caller.
  * @throws AKERR_NULLPOINTER If @p font or @p text is `NULL`; if the global
- *         `renderer`, its `sdl_renderer`, or its `draw_texture` is `NULL` --
+ *         `akgl_renderer`, its `sdl_renderer`, or its `draw_texture` is `NULL` --
  *         that last one is the state a backend is in between being allocated
- *         and being run through akgl_render_bind2d(); if SDL_ttf cannot
+ *         and being run through akgl_render_2d_bind(); if SDL_ttf cannot
  *         rasterize the string; or if the surface cannot be uploaded as a
  *         texture. The last two carry `SDL_GetError()` and are a reused status
  *         rather than a pointer problem.
@@ -155,4 +155,4 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_text_measure(TTF_Font *font, char *text,
  */
 akerr_ErrorContext AKERR_NOIGNORE *akgl_text_measure_wrapped(TTF_Font *font, char *text, int wraplength, int *w, int *h);
 
-#endif // _TEXT_H_
+#endif // _AKGL_TEXT_H_