Reach hardware through backend records, and take the time from the host

Groups G, I and E are unblocked but cannot be written yet: the core library is
free of SDL and builds with no libakgl present, so a graphics verb cannot call
akgl_draw_* and a sound verb cannot call akgl_audio_*. This adds what they call
instead.

Three records of function pointers -- akbasic_GraphicsBackend, _AudioBackend and
_InputBackend -- in the same shape as akbasic_TextSink, and the same shape
libakgl uses for akgl_RenderBackend. akbasic_runtime_set_devices() attaches any
subset; all three may be NULL and that is the standalone driver's normal state,
so a runtime with no backends still comes up and still prints. A verb that needs
one it was not given raises the new AKBASIC_ERR_DEVICE rather than dereferencing
a NULL vtable.

Two decisions worth stating. The graphics record has no circle entry point:
BASIC 7.0's CIRCLE takes two radii, an arc range, a rotation and a degree
increment, which makes it a polygon by definition, so it will be built from line
calls rather than from akgl_draw_circle. And coordinates are double rather than
an integer pixel address, because SCALE makes them fractional and rounding at
each verb rather than once at the backend accumulates drift along a polyline.

akbasic_runtime_settime() is how SOUND, PLAY and TEMPO get a clock without the
library reading one. Section 1.6 forbids blocking or owning a loop, so the caller
that owns the frame owns the time -- which is what libakgl already does, since
akgl_actor_logic_changeframe takes curtimems as an argument. Left unset it is
zero and every duration expires immediately: audible, but never a hang.

AKBASIC_ERR_LAST is a sentinel rather than a status, so tests/error_codes.c walks
every code looking for an unnamed one without anybody remembering to widen the
loop when a code is added.

tests/mockdevice.h records every backend call as a formatted line. The graphics
and audio verbs emit nothing a golden file can compare, so that log is where
their assertions have to live -- and since it needs no SDL, the whole of groups
G, I and E stays testable in the default build.

62/62 ctest, clean under -Wall -Wextra, doxygen clean.

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

include/akbasic/audio.h

@@ -0,0 +1,71 @@
+/**
+ * @file audio.h
+ * @brief Declares the audio backend: where SOUND, PLAY, ENVELOPE and VOL land.
+ *
+ * Same reasoning as graphics.h -- the core library is free of SDL, so the sound
+ * verbs call through a record of function pointers and akbasic_audio_init_akgl()
+ * in the akbasic_akgl target wires that record to akgl_audio_*.
+ *
+ * The libakgl API this is modelled on is a tone generator, not a sample player:
+ * three voices, a waveform and an ADSR envelope each, mixed to one stream. That
+ * is the right shape, because BASIC 7.0's sound verbs describe notes rather than
+ * recordings. What it does not have is a filter stage, which is why FILTER is
+ * refused -- see TODO.md section 7.
+ */
+
+#ifndef _AKBASIC_AUDIO_H_
+#define _AKBASIC_AUDIO_H_
+
+#include <akerror.h>
+
+#include <akbasic/types.h>
+
+/** @brief Number of independent voices, matching the SID and matching libakgl. */
+#define AKBASIC_AUDIO_VOICES	3
+
+/**
+ * @brief Waveform selection, numbered as BASIC 7.0's SOUND numbers it.
+ *
+ * Declared here rather than reused from akgl/audio.h so the core library does
+ * not include a libakgl header. The akgl backend maps these across; the values
+ * happen to agree today and the mapping is still written out, because two
+ * enumerations agreeing by accident is not a contract.
+ */
+typedef enum
+{
+    AKBASIC_WAVE_TRIANGLE = 0,	/* SOUND waveform 0 */
+    AKBASIC_WAVE_SAWTOOTH = 1,	/* SOUND waveform 1 */
+    AKBASIC_WAVE_SQUARE	  = 2,	/* SOUND waveform 2, and the power-on default */
+    AKBASIC_WAVE_NOISE	  = 3	/* SOUND waveform 3 */
+} akbasic_Waveform;
+
+/**
+ * @brief Where the sound verbs play.
+ *
+ * Voices here are numbered from zero. BASIC numbers them 1 through 3 and the
+ * conversion happens once, in the verb handler, so a backend never sees a BASIC
+ * voice number.
+ *
+ * Durations are milliseconds. BASIC counts in jiffies (1/60 s) and note lengths
+ * derived from TEMPO; both are converted in src/audio_tables.c before they get
+ * here, so a backend never sees a jiffy either.
+ */
+typedef struct akbasic_AudioBackend
+{
+    void *self;
+
+    /** 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);
+    /** Silence a voice immediately. */
+    akerr_ErrorContext AKERR_NOIGNORE *(*stop)(struct akbasic_AudioBackend *self, int voice);
+    /** Select the waveform a voice synthesises. */
+    akerr_ErrorContext AKERR_NOIGNORE *(*waveform)(struct akbasic_AudioBackend *self, int voice, akbasic_Waveform waveform);
+    /** Set a voice's ADSR envelope. Sustain is a level from 0.0 to 1.0, not a time. */
+    akerr_ErrorContext AKERR_NOIGNORE *(*envelope)(struct akbasic_AudioBackend *self, int voice, int attack, int decay, double sustain, int release);
+    /** Set the master output level, 0.0 to 1.0. */
+    akerr_ErrorContext AKERR_NOIGNORE *(*volume)(struct akbasic_AudioBackend *self, double level);
+    /** Report whether a voice is still sounding. PLAY's queue uses this to pace itself. */
+    akerr_ErrorContext AKERR_NOIGNORE *(*voice_active)(struct akbasic_AudioBackend *self, int voice, bool *active);
+} akbasic_AudioBackend;
+
+#endif // _AKBASIC_AUDIO_H_