Synthesise tones on three voices

There was no audio API at all: SDL3_mixer was vendored and AKGL_REGISTRY_MUSIC
was declared, but nothing opened a mixer, and a mixer plays recordings anyway.
BASIC 7.0's SOUND, PLAY, ENVELOPE and VOL describe notes, so this adds a tone
generator -- three voices, five waveforms, an ADSR envelope each, mixed to one
float stream and fed to an SDL_AudioStream.

The voice table works whether or not a device is open. akgl_audio_init connects
it to one; a host that owns its own audio pipeline can call akgl_audio_mix
instead. That is also what makes the tests deterministic: a device pulls samples
on SDL's audio thread whenever it likes, so tests/audio.c mixes by hand and only
opens a device in its last test.

Phase is computed from the frame counter rather than accumulated, because a
float increment of hz/44100 added 44100 times a second walks a held note off
pitch. A voice that has never been configured defaults to a square wave at full
level, since a zeroed voice would have a sustain of 0.0 and make no sound with
no error to explain why. Voices summing past full scale are clamped rather than
scaled, so one voice plays at the level it was asked for.

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

CMakeLists.txt

@@ -109,6 +109,7 @@ add_library(akgl SHARED
   deps/semver/semver.c
   src/actor.c
   src/actor_state_string_names.c
+  src/audio.c
   src/text.c
   src/assets.c
   src/character.c
@@ -154,6 +155,7 @@ add_executable(akgl_test_semver_unit deps/semver/semver_unit.c)
 # akgl_test_<name> and registered with CTest under <name>.
 set(AKGL_TEST_SUITES
   actor
+  audio
   bitmasks
   character
   controller
@@ -333,6 +335,7 @@ install(FILES "include/akgl/actor.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/types.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/text.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/assets.h" DESTINATION "include/akgl/")
+install(FILES "include/akgl/audio.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/character.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/error.h" DESTINATION "include/akgl/")
 install(FILES "include/akgl/draw.h" DESTINATION "include/akgl/")