Close the test gaps mutation testing found in audio and draw

A mutation smoke run scored src/audio.c at 50% and src/draw.c at 70% and named
three real holes:

- Nothing asserted that an unconfigured voice is audible, which is the entire
  reason the voice table defaults to a square wave at full level instead of a
  zeroed struct. Deleting the defaults survived.
- No envelope test used a non-zero attack and decay together, so the decay
  measuring from the start of the note rather than from the end of the attack
  survived.
- The circle was only checked at its four axis points, which a mis-signed
  octant reflection survives. It now checks that every plotted pixel has a
  mirror in the other three quadrants.

Also adds a backend that exists but was never given an SDL_Renderer, which is
the state a host is in between allocating one and initializing it; every draw
entry point has to report it rather than dereference it.

audio 50% -> 75%, draw 70% -> 90%. The survivors are recorded in TODO.md and
are honestly untestable from here.

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

tests/audio.c

@@ -23,8 +23,13 @@
 
 #include "testutil.h"
 
-/** @brief Frames of output most tests generate at a time. */
-#define TEST_MIX_FRAMES 512
+/**
+ * @brief Frames of output most tests generate at a time.
+ *
+ * Enough to hold a 10 ms attack and a 10 ms decay back to back at 44100 frames
+ * per second, which is the longest single stretch any test here inspects.
+ */
+#define TEST_MIX_FRAMES 1024
 
 /** @brief 441 Hz at 44100 frames per second is exactly 100 frames per cycle. */
 #define TEST_TONE_HZ 441.0f
@@ -67,6 +72,30 @@ static float32_t peak_of(int frames)
     return peak;
 }
 
+akerr_ErrorContext *test_audio_defaults(void)
+{
+    PREPARE_ERROR(errctx);
+
+    ATTEMPT {
+	// Deliberately the first thing this file does, and deliberately without
+	// a call to reset_audio(): a voice nobody has configured has to make a
+	// sound. A zeroed voice would have a sustain of 0.0 and be silent, with
+	// no error to say why, which is exactly the trap this defends against.
+	CATCH(errctx, akgl_audio_tone(0, TEST_TONE_HZ, 1000));
+	CATCH(errctx, akgl_audio_mix(samples, TEST_MIX_FRAMES));
+	TEST_ASSERT_FEQ(errctx, samples[0], 1.0f,
+			"an unconfigured voice mixed to %f, expected a full-level square wave",
+			samples[0]);
+	TEST_ASSERT_FEQ(errctx, samples[50], -1.0f,
+			"an unconfigured voice is not a square wave (%f half a cycle in)",
+			samples[50]);
+    } CLEANUP {
+	IGNORE(reset_audio());
+    } PROCESS(errctx) {
+    } FINISH(errctx, true);
+    SUCCEED_RETURN(errctx);
+}
+
 akerr_ErrorContext *test_audio_silence(void)
 {
     PREPARE_ERROR(errctx);
@@ -254,6 +283,21 @@ akerr_ErrorContext *test_audio_envelope(void)
 	TEST_ASSERT_FEQ(errctx, samples[450], -0.5f,
 			"after the decay the level is %f, expected the 0.5 sustain", samples[450]);
 
+	// Attack and decay together, which is the case where the decay has to
+	// measure from the end of the attack rather than from the start of the
+	// note. 10 ms of each is 441 frames of each at 44100.
+	CATCH(errctx, reset_audio());
+	CATCH(errctx, akgl_audio_envelope(0, 10, 10, 0.5f, 0));
+	CATCH(errctx, akgl_audio_tone(0, TEST_TONE_HZ, 1000));
+	CATCH(errctx, akgl_audio_mix(samples, TEST_MIX_FRAMES));
+	TEST_ASSERT_FEQ(errctx, samples[441], 1.0f,
+			"at the end of the attack the level is %f, expected full", samples[441]);
+	TEST_ASSERT_FEQ(errctx, samples[661], -(1.0f - (0.5f * (220.0f / 441.0f))),
+			"halfway through the decay the level is %f, expected the halfway ramp",
+			samples[661]);
+	TEST_ASSERT_FEQ(errctx, samples[900], 0.5f,
+			"after the decay the level is %f, expected the 0.5 sustain", samples[900]);
+
 	TEST_EXPECT_STATUS(errctx, AKERR_OUTOFBOUNDS, akgl_audio_envelope(0, 1, 1, -0.1f, 1),
 			   "setting a negative sustain level");
 	TEST_EXPECT_STATUS(errctx, AKERR_OUTOFBOUNDS, akgl_audio_envelope(0, 1, 1, 1.5f, 1),
@@ -432,6 +476,7 @@ int main(void)
 	    "Couldn't initialize SDL: %s",
 	    SDL_GetError());
 
+	CATCH(errctx, test_audio_defaults());
 	CATCH(errctx, test_audio_silence());
 	CATCH(errctx, test_audio_square_tone());
 	CATCH(errctx, test_audio_waveforms());