Record the UI's paper trail and bump to 0.9.0

Two TODO.md notes the UI work created. The text texture ring cache (plan
item 2) now has its second consumer, and the bigger one: the UI executor
draws every TEXT render command through akgl_text_rendertextat, one wrapped
line per command per frame, so a five-row menu re-rasterizes five lines at
60 Hz against the original case of one HUD line changing once a second.
And akgl_ControlMap's mouseid/penid fields stay dead deliberately -- the
mouse path lives in the UI subsystem because control maps and pointer state
are different consumers, and the note says where the work lands when a game
wants mouse-driven actors.

0.8.0 to 0.9.0: a new public header, a new status code, three new drawing
primitives and a new subsystem is new API, and while the major version is 0
the soname carries major.minor -- libakgl.so.0.9 is what makes 0.8.0 and
0.9.0 unmixable at load time, which is the honesty the bump is for.

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

CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.10)
 # The single source of truth for the library version. It drives the generated
 # include/akgl/version.h, the shared library's VERSION and SOVERSION, and the
 # Version field in akgl.pc. Bump it here and nowhere else.
-project(akgl VERSION 0.8.0 LANGUAGES C)
+project(akgl VERSION 0.9.0 LANGUAGES C)
 
 # Memory checking reuses the suites that already exist -- `ctest -T memcheck`
 # runs every registered test under valgrind -- rather than adding programs of its