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>

CMakeLists.txt

@@ -217,6 +217,7 @@ set(AKGL_PUBLIC_HEADERS
   text
   tilemap
   types
+  ui
   util
 )
 
@@ -317,6 +318,8 @@ add_library(akgl SHARED
   src/sprite.c
   src/staticstring.c
   src/tilemap.c
+  src/ui.c
+  src/ui_clay.c
   src/util.c
   src/version.c
 )
@@ -368,6 +371,26 @@ set_source_files_properties(${AKGL_CCD_SOURCES} PROPERTIES
   COMPILE_OPTIONS "-w;-fvisibility=hidden;-include;${CMAKE_CURRENT_SOURCE_DIR}/src/ccd_arena_shim.h"
   COMPILE_DEFINITIONS "CCD_STATIC_DEFINE")
 
+# clay supplies the UI layout engine. Its sources are listed rather than
+# add_subdirectory()'d, on the semver/libccd precedent, because
+# deps/clay/CMakeLists.txt is unusable as a subproject and none of it is ours
+# to fix in a submodule:
+#
+#   - it declares no library target at all -- clay is a single header, and the
+#     add_library(INTERFACE) lines at the bottom are commented out upstream.
+#   - option(CLAY_INCLUDE_ALL_EXAMPLES ... ON) add_subdirectory()s every
+#     example by default, dragging raylib, cairo and sokol into our configure.
+#   - cmake_minimum_required(VERSION 3.27) against this project's 3.10.
+#
+# src/ui_clay.c is the one translation unit that defines CLAY_IMPLEMENTATION;
+# it is 99% vendored code, so it gets -w on the same terms as semver and
+# libccd. Deliberately NOT -fvisibility=hidden, unlike libccd: the CLAY()
+# macros in a consuming game expand to Clay__OpenElement() and friends, which
+# must resolve against libakgl.so, so clay's symbols are part of the ABI on
+# purpose. The matching obligation -- a consumer must not link a second clay --
+# is documented in akgl/ui.h.
+set_source_files_properties(src/ui_clay.c PROPERTIES COMPILE_OPTIONS "-w")
+
 # PRIVATE, so that ccd/*.h never reaches a consumer's include path. The `headers`
 # suite compiles each public header as the first include of a translation unit
 # with only include/ available, so a public header that pulled in <ccd/ccd.h>
@@ -406,6 +429,7 @@ set(AKGL_TEST_SUITES
   staticstring
   text
   tilemap
+  ui
   util
   version
 )
@@ -516,6 +540,11 @@ set_tests_properties(
 target_include_directories(akgl PUBLIC
   include/
   deps/semver/
+  # PUBLIC, unlike libccd's PRIVATE include above, because akgl/ui.h includes
+  # <clay.h> in its public interface -- consumers write CLAY() blocks, so
+  # hiding the header would hide the point. clay.h is installed beside
+  # semver.h for the same reason.
+  deps/clay/
   # akgl/version.h is generated by configure_file, so it lives in the build tree
   # rather than beside the headers it is included from.
   ${CMAKE_CURRENT_BINARY_DIR}/include/
@@ -882,11 +911,14 @@ install(FILES ${CMAKE_CURRENT_BINARY_DIR}/akgl.pc DESTINATION "lib/pkgconfig/")
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/include/akgl/version.h DESTINATION "include/akgl/")
 install(TARGETS akgl DESTINATION "lib/")
 install(FILES "deps/semver/semver.h" DESTINATION "include/")
-# libccd is compiled into libakgl.so, and its BSD-3 licence requires the notice
-# to travel with the binary form. Nothing else in deps/ is linked in statically
-# -- SDL, jansson, libakerror and libakstdlib are all separate shared objects
-# that ship their own -- so this is the only third-party notice we owe.
+install(FILES "deps/clay/clay.h" DESTINATION "include/")
+# libccd and clay are compiled into libakgl.so, and their licences (BSD-3 and
+# zlib respectively) ask that the notice travel with the binary form. Nothing
+# else in deps/ is linked in statically -- SDL, jansson, libakerror and
+# libakstdlib are all separate shared objects that ship their own -- so these
+# are the only third-party notices we owe.
 install(FILES "deps/libccd/BSD-LICENSE" DESTINATION "share/doc/akgl/" RENAME "BSD-LICENSE.libccd")
+install(FILES "deps/clay/LICENSE.md" DESTINATION "share/doc/akgl/" RENAME "LICENSE.clay")
 foreach(header IN LISTS AKGL_PUBLIC_HEADERS)
   install(FILES "include/akgl/${header}.h" DESTINATION "include/akgl/")
 endforeach()