Consume libakgl 0.3.0: every workaround deleted, two capabilities gained

0.3.0 closed all ten API gaps this port had filed. The four workarounds go
with them: the CMake block that declared libakgl's vendored dependencies by
hand, the akgl/actor.h include in three files, and the six vtable pointers
assigned by hand in two more, now akgl_render_bind2d().

Two gaps were capabilities rather than inconveniences, and both are now real:

The line editor takes the composed UTF-8 text the ring carries in preference
to the keycode, so shifted characters, keyboard layouts, compose keys and dead
keys all work. A double quote can be typed, which means a BASIC string literal
can be typed -- the sharp end of the old limitation. Letters are no longer
folded to upper case.

SOUND's dir/min/step reach akgl_audio_sweep instead of being refused. dir 3
sweeps once rather than oscillating and TODO.md section 5 says so. A backend
with no sweep still refuses the swept note and plays the held one.

The adaptors now carry an AKGL_VERSION_AT_LEAST(0, 3, 0) floor, verified by
temporarily demanding 0.4.0 and watching it fire.

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

include/akbasic/audio.h

@@ -67,6 +67,16 @@ typedef struct akbasic_AudioBackend
 
     /** Start a note on a voice, for a bounded duration. */
     akerr_ErrorContext AKERR_NOIGNORE *(*tone)(struct akbasic_AudioBackend *self, int voice, double hz, int ms);
+    /**
+     * Start a note whose pitch moves, which is what SOUND's `dir`, `min` and
+     * `step` arguments ask for -- a siren, a laser, a falling bomb.
+     *
+     * May be NULL, and a host that leaves it so gets `SOUND` refused with
+     * AKBASIC_ERR_DEVICE for a swept note and served normally for a held one.
+     * That is deliberate: this arrived in libakgl 0.3.0, and a backend written
+     * against an older one is still a valid backend for everything else.
+     */
+    akerr_ErrorContext AKERR_NOIGNORE *(*sweep)(struct akbasic_AudioBackend *self, int voice, double from_hz, double to_hz, double step_hz, int ms);
     /** Silence a voice immediately. */
     akerr_ErrorContext AKERR_NOIGNORE *(*stop)(struct akbasic_AudioBackend *self, int voice);
     /** Select the waveform a voice synthesises. */