Record the akbasic API gaps as resolved and bump to 0.3.0

All ten items under "API gaps blocking akbasic" are closed. Items 7, 9 and
10 add public symbols and item 9 adds three fields to akgl_AudioVoice, so
the version and the soname go with them -- an 0.2 consumer cannot be handed
this library and told it is compatible.

Also refreshes the coverage numbers against a fresh run (79.6% line, 87.2%
function, all 21 suites), records the mutation smoke runs over the two files
that changed most, and files one new defect the text tests turned up:
akgl_text_rendertextat refuses the empty string while akgl_text_measure
accepts it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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 0.2.0 LANGUAGES C)
+project(akgl VERSION 0.3.0 LANGUAGES C)
 
 include(CTest)
 option(AKGL_COVERAGE "Instrument libakgl and generate coverage reports with CTest" OFF)