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>

src/runtime.c

@@ -152,6 +152,41 @@ akerr_ErrorContext *akbasic_runtime_init(akbasic_Runtime *obj, akbasic_TextSink
     SUCCEED_RETURN(errctx);
 }
 
+akerr_ErrorContext *akbasic_runtime_set_devices(akbasic_Runtime *obj, akbasic_GraphicsBackend *graphics, akbasic_AudioBackend *audio, akbasic_InputBackend *input)
+{
+    PREPARE_ERROR(errctx);
+
+    FAIL_ZERO_RETURN(errctx, (obj != NULL), AKERR_NULLPOINTER,
+		     "NULL runtime in set_devices");
+
+    /*
+     * No validation of the records themselves. A backend with a NULL entry point
+     * is caught at the call site by the verb that needs it, which can say which
+     * verb wanted what -- checking every pointer here would only be able to say
+     * that something, somewhere, was incomplete.
+     */
+    obj->graphics = graphics;
+    obj->audio = audio;
+    obj->input = input;
+    SUCCEED_RETURN(errctx);
+}
+
+akerr_ErrorContext *akbasic_runtime_settime(akbasic_Runtime *obj, int64_t timems)
+{
+    PREPARE_ERROR(errctx);
+
+    FAIL_ZERO_RETURN(errctx, (obj != NULL), AKERR_NULLPOINTER,
+		     "NULL runtime in settime");
+
+    /*
+     * Deliberately not rejecting a time that moves backwards. A host is free to
+     * drive this from a paused, scrubbed or replayed clock, and the only thing
+     * that happens is a note holding longer than it asked to.
+     */
+    obj->timems = timems;
+    SUCCEED_RETURN(errctx);
+}
+
 /* ---------------------------------------------------------------- output -- */
 
 akerr_ErrorContext *akbasic_runtime_write(akbasic_Runtime *obj, const char *text)