Vendor clay and bring the UI subsystem up: arena, lifecycle, status band

clay v0.14 (deps/clay, zlib licence) supplies the layout engine for the new
akgl_ui subsystem. Sources-listed rather than add_subdirectory()'d, on the
semver/libccd precedent: clay's CMakeLists declares no library target, builds
its examples by default, and requires CMake 3.27 against this project's 3.10.
src/ui_clay.c is the one CLAY_IMPLEMENTATION translation unit, compiled -w on
the vendored-code terms but with clay's symbols deliberately exported --
consumers' CLAY() macros resolve against libakgl.so, and akgl/ui.h warns them
never to link a second clay.

The subsystem is runtime-optional the way collision is: always compiled in,
inert until akgl_ui_init(), which bounds clay to the overridable AKGL_UI_*
ceilings, checks Clay_MinMemorySize() against a static BSS arena (no malloc),
and refuses with both numbers in the message when it does not fit. Measured:
812544 bytes at the default 1024 elements / 4096 measured words, against a
1 MiB arena. clay's void-callback layout errors are logged as they happen and
the first is stashed for the error protocol to raise later.

AKGL_ERR_UI joins the status band before AKGL_ERR_LIMIT, named in
akgl_error_init. New `ui` test suite covers validation, the init/shutdown
lifecycle, and arena exhaustion via the akgl_ui_arena_limit test hook (same
contract as akgl_ccd_arena_set_limit). clay.h is installed beside semver.h,
its licence beside libccd's, and the headers suite proves akgl/ui.h
self-contained -- clay.h compiles clean under -Wall.

Co-Authored-By: Claude Code (Claude Fable 5, claude-fable-5) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

include/akgl/error.h

@@ -76,11 +76,12 @@
 #define AKGL_ERR_BEHAVIOR          (AKGL_ERR_BASE + 3)   /**< A component did not behave the way its contract requires */
 #define AKGL_ERR_LOGICINTERRUPT    (AKGL_ERR_BASE + 4)   /**< Actor logic is telling the physics simulator to skip it */
 #define AKGL_ERR_COLLISION         (AKGL_ERR_BASE + 5)   /**< A collision query could not be answered; the message says why */
+#define AKGL_ERR_UI                (AKGL_ERR_BASE + 6)   /**< The UI subsystem refused, or clay reported a layout error; the message says which */
 
 // One past the last libakgl status. The reservation is all-or-nothing -- a
 // subset or superset of an existing one is refused -- so this must stay one
 // past the highest code above.
-#define AKGL_ERR_LIMIT             (AKGL_ERR_BASE + 6)
+#define AKGL_ERR_LIMIT             (AKGL_ERR_BASE + 7)
 #define AKGL_ERR_COUNT             (AKGL_ERR_LIMIT - AKGL_ERR_BASE)
 
 /**