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
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

src/error.c

@@ -21,5 +21,6 @@ akerr_ErrorContext *akgl_error_init(void)
     PASS(errctx, akerr_register_status_name(AKGL_ERR_OWNER, AKGL_ERR_BEHAVIOR, "Behavior Error"));
     PASS(errctx, akerr_register_status_name(AKGL_ERR_OWNER, AKGL_ERR_LOGICINTERRUPT, "Logic Interrupt"));
     PASS(errctx, akerr_register_status_name(AKGL_ERR_OWNER, AKGL_ERR_COLLISION, "Collision Error"));
+    PASS(errctx, akerr_register_status_name(AKGL_ERR_OWNER, AKGL_ERR_UI, "UI Error"));
     SUCCEED_RETURN(errctx);
 }

src/ui.c

@@ -0,0 +1,153 @@
+/**
+ * @file ui.c
+ * @brief Implements the UI subsystem: clay's arena, lifecycle and error stash.
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <akerror.h>
+#include <akstdlib.h>
+#include <clay.h>
+#include <SDL3/SDL.h>
+
+#include <akgl/error.h>
+#include <akgl/ui.h>
+
+/**
+ * @brief Bytes of the first clay error's text kept for reporting.
+ *
+ * Sized to hold every message clay 0.14 actually emits -- they are one
+ * sentence each -- with room for a longer one to truncate into rather than
+ * overflow.
+ */
+#define UI_ERROR_TEXT_LENGTH    256
+
+/** @brief The arena clay lays its structures out in. Static storage, so this file allocates nothing. */
+static uint8_t ui_arena_bytes[AKGL_UI_ARENA_BYTES];
+/** @brief Usable bytes. #AKGL_UI_ARENA_BYTES unless a test has narrowed it. */
+static size_t ui_arena_usable = AKGL_UI_ARENA_BYTES;
+/** @brief The context Clay_Initialize handed back, or NULL before init / after shutdown. */
+static Clay_Context *ui_context;
+
+/**
+ * @brief First error clay reported since the stash was last cleared.
+ *
+ * clay's error handler is a void callback with no way to refuse or return, so
+ * a layout error cannot surface at the call that caused it -- it fires in the
+ * middle of Clay_EndLayout with libakgl nowhere on the stack. The handler
+ * stashes the first message here and akgl_ui_frame_end raises it, which is
+ * the earliest point the error protocol can carry it. Later errors in the
+ * same frame are counted but not kept: the first one is almost always the
+ * cause and the rest its consequences.
+ */
+static char ui_clay_error_text[UI_ERROR_TEXT_LENGTH];
+/** @brief How many errors clay has reported since the stash was cleared. */
+static uint32_t ui_clay_error_count;
+
+/**
+ * @brief The error handler given to Clay_Initialize.
+ *
+ * Logs every report as it happens -- an operator watching the log should not
+ * have to wait for frame_end -- and stashes the first for the error protocol.
+ * Clay_String is a length and a pointer, not a C string, so the copy is
+ * bounded by hand before aksl_strncpy sees it.
+ */
+static void ui_clay_error(Clay_ErrorData error)
+{
+    size_t length = 0;
+
+    SDL_Log("clay: %.*s", (int)error.errorText.length, error.errorText.chars);
+    ui_clay_error_count += 1;
+    if ( ui_clay_error_count > 1 ) {
+	return;
+    }
+    length = (size_t)error.errorText.length;
+    if ( length > (sizeof(ui_clay_error_text) - 1) ) {
+	length = sizeof(ui_clay_error_text) - 1;
+    }
+    IGNORE(aksl_strncpy(ui_clay_error_text, sizeof(ui_clay_error_text), error.errorText.chars, length));
+}
+
+akerr_ErrorContext *akgl_ui_init(int width, int height)
+{
+    uint32_t needed = 0;
+    Clay_Arena arena;
+    Clay_ErrorHandler handler = { &ui_clay_error, NULL };
+    Clay_Dimensions dimensions;
+
+    PREPARE_ERROR(errctx);
+    FAIL_NONZERO_RETURN(errctx, (width <= 0), AKERR_OUTOFBOUNDS, "Layout width %d is not positive", width);
+    FAIL_NONZERO_RETURN(errctx, (height <= 0), AKERR_OUTOFBOUNDS, "Layout height %d is not positive", height);
+    FAIL_NONZERO_RETURN(
+	errctx,
+	(ui_context != NULL),
+	AKGL_ERR_UI,
+	"The UI subsystem is already initialized; akgl_ui_shutdown first");
+
+    // Ceilings before measurement: Clay_MinMemorySize reports what the
+    // *current* ceilings need, so they must be set first. Element count
+    // before word count, and not the other way around -- with no context
+    // yet, Clay_SetMaxElementCount also overwrites the word-cache default
+    // with elements * 2, and would silently undo a word count set before it.
+    Clay_SetMaxElementCount(AKGL_UI_MAX_ELEMENTS);
+    Clay_SetMaxMeasureTextCacheWordCount(AKGL_UI_MAX_MEASURE_WORDS);
+    needed = Clay_MinMemorySize();
+    FAIL_NONZERO_RETURN(
+	errctx,
+	((size_t)needed > ui_arena_usable),
+	AKGL_ERR_UI,
+	"clay needs %u bytes for %d elements and %d measured words; the arena holds %u. Raise AKGL_UI_ARENA_BYTES.",
+	needed,
+	AKGL_UI_MAX_ELEMENTS,
+	AKGL_UI_MAX_MEASURE_WORDS,
+	(uint32_t)ui_arena_usable);
+
+    ui_clay_error_text[0] = '\0';
+    ui_clay_error_count = 0;
+    arena = Clay_CreateArenaWithCapacityAndMemory(ui_arena_usable, ui_arena_bytes);
+    dimensions.width = (float)width;
+    dimensions.height = (float)height;
+    ui_context = Clay_Initialize(arena, dimensions, handler);
+    // The capacity check above is the one that should fail; this one exists
+    // because Clay_Initialize reports refusal by returning NULL and a NULL
+    // context would otherwise surface as a crash inside the first CLAY()
+    // block, far from the cause.
+    FAIL_ZERO_RETURN(errctx, ui_context, AKGL_ERR_UI, "Clay_Initialize refused: %s", ui_clay_error_text);
+    SUCCEED_RETURN(errctx);
+}
+
+akerr_ErrorContext *akgl_ui_shutdown(void)
+{
+    PREPARE_ERROR(errctx);
+    // clay holds no resource but the arena, and the arena is ours: there is
+    // nothing to hand back, so shutdown is forgetting. The next init lays a
+    // fresh context over the same bytes.
+    ui_context = NULL;
+    ui_clay_error_text[0] = '\0';
+    ui_clay_error_count = 0;
+    SUCCEED_RETURN(errctx);
+}
+
+akerr_ErrorContext *akgl_ui_resize(int width, int height)
+{
+    Clay_Dimensions dimensions;
+
+    PREPARE_ERROR(errctx);
+    FAIL_ZERO_RETURN(errctx, ui_context, AKGL_ERR_UI, "The UI subsystem is not initialized");
+    FAIL_NONZERO_RETURN(errctx, (width <= 0), AKERR_OUTOFBOUNDS, "Layout width %d is not positive", width);
+    FAIL_NONZERO_RETURN(errctx, (height <= 0), AKERR_OUTOFBOUNDS, "Layout height %d is not positive", height);
+    dimensions.width = (float)width;
+    dimensions.height = (float)height;
+    Clay_SetLayoutDimensions(dimensions);
+    SUCCEED_RETURN(errctx);
+}
+
+void akgl_ui_arena_limit(size_t limit)
+{
+    if ( (limit == 0) || (limit > AKGL_UI_ARENA_BYTES) ) {
+	ui_arena_usable = AKGL_UI_ARENA_BYTES;
+    } else {
+	ui_arena_usable = limit;
+    }
+}

src/ui_clay.c

@@ -0,0 +1,21 @@
+/**
+ * @file ui_clay.c
+ * @brief The one translation unit that compiles clay's implementation.
+ *
+ * clay is a single-header library: every other file that includes <clay.h>
+ * gets declarations only, and exactly one file in the whole program defines
+ * CLAY_IMPLEMENTATION before the include to get the definitions. This is that
+ * file, and it must stay the only one -- a second definition anywhere,
+ * including in a consuming game, is two copies of clay's globals and the
+ * loader picking between them silently. The warning in akgl/ui.h says so to
+ * consumers; this comment says so to us.
+ *
+ * Nothing else belongs here. libakgl's own UI code is src/ui.c, which
+ * includes <clay.h> like any other consumer. CMakeLists.txt compiles this
+ * file with -w on the same terms as deps/semver and deps/libccd: 99% of what
+ * this TU contains is vendored code, and a future clay bump must not be able
+ * to fail the build on a warning we do not own.
+ */
+
+#define CLAY_IMPLEMENTATION
+#include <clay.h>