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>

TODO.md

@@ -407,12 +407,39 @@ came from elsewhere in the tree and this note was never updated.)
 | `src/util.c` | 121/131 (92%) | 7/8 |
 | `src/version.c` | 2/2 (100%) | 1/1 |
 
-Branch coverage reads 20.8% and should not be used as a target. The akerror
+Branch coverage reads 21.2% and should not be used as a target. The akerror
 control-flow macros (`ATTEMPT`/`CATCH`/`PROCESS`/`FINISH`, `FAIL_*_RETURN`)
 expand into large branch trees per call site, most of them unreachable in normal
 operation — `src/game.c` reports over 1700 branches across 230 lines. Track line
 and function coverage; treat branch coverage as a relative signal within a file.
 
+### Mutation testing
+
+`scripts/mutation_test.py` was run over the three new files as a smoke check
+(`--max-mutants 8` to 10 each, so these are samples rather than exhaustive
+scores):
+
+| File | Score | Surviving mutants |
+|---|---|---|
+| `src/draw.c` | 90% | Deleting the `FAIL_ZERO_BREAK` on `SDL_CreateTextureFromSurface` in `akgl_draw_flood_fill` |
+| `src/audio.c` | 75% | Deleting `SUCCEED_RETURN` from the static `check_voice`; deleting `spec.freq` before opening a device |
+| `src/text.c` | 50% | Three in `akgl_text_rendertextat`, which has no test yet, plus one `SUCCEED_RETURN` deletion |
+
+The first pass over `src/audio.c` scored 50% and named two real gaps, both of
+which are now tested: nothing asserted that an *unconfigured* voice is audible
+(the whole reason the table defaults to a square wave at full level rather than
+a zeroed struct), and no envelope test used a non-zero attack *and* decay
+together, so the decay measuring from the wrong origin survived. `src/draw.c`
+scored 70% first and named one: the circle was only checked at its four axis
+points, which a mis-signed octant reflection survives, so it now checks that
+every plotted pixel has a mirror in the other three quadrants.
+
+What is left is honestly untestable from here. Deleting a `SUCCEED_RETURN`
+leaves a non-void function falling off its end, which is undefined rather than
+observably wrong, and the surviving SDL branches are allocation failures the
+suite has no way to provoke. The `src/text.c` survivors go away with the
+offscreen harness.
+
 ### Suites
 
 Every suite is registered through the `AKGL_TEST_SUITES` list in
@@ -634,6 +661,12 @@ Each was found by a test written to assert correct behavior.
     The `build*/` entry in `.gitignore` hides these trees from `git status`,
     which makes the state easier to get into and no easier to notice.
 
+    A second, smaller version of the same thing: rebuilding an existing
+    coverage tree after editing a test leaves `.gcda` files describing the old
+    object layout, and `coverage_reset` -- whose whole job is to delete them --
+    fails with `GCOV returncode was 5` before it gets the chance. `find
+    build-coverage -name '*.gcda' -delete` clears it. Observed with gcovr 7.0.
+
     Fix: pass the build tree to gcovr as an explicit search path instead of
     letting it default to `--root`, so only the tree under measurement is
     considered. Touches the two `add_test` blocks at `CMakeLists.txt:204-211`