fix(audio): zero-initialise the channel layouts before the resampler copy

av_channel_layout_copy() documents that it always uninitialises the
destination first, and av_channel_layout_uninit() calls av_freep() on
u.map. Declaring the layouts without {} therefore handed free() whatever
pointer-shaped garbage occupied that stack slot.

Not theoretical: UT-103 aborted with "free(): invalid pointer" in about
1 run in 4. Zero failures in 40 runs after the fix, against 10 in 40
before it.

Two things kept it hidden, both worth remembering:

- It is stack-dependent, so it disappears under an AddressSanitizer
  build and reads as a flake in the aggregate test binary, where the
  case usually passes. ctest, which runs each case in its own process,
  is what made it a reproducible failure rather than noise.
- UT-103 is the only test that reaches this branch, because it is the
  only one whose input is stereo. The golden-vector tests use a mono
  11025 Hz fixture chosen so the vector cannot depend on libswresample
  — which is right, and means bit-exactness against the golden vector
  is not evidence about the downmix path.

TRACES: IR-004 | SR-003

src/audio_signature.cpp

@@ -155,9 +155,24 @@ struct DecodeCtx {
 
 bool open_resampler(DecodeCtx& c, const AVFrame* f) {
 #if LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(57, 24, 100)
-    AVChannelLayout out_layout;
+    // Both MUST be zero-initialised. av_channel_layout_copy documents that it
+    // "will always uninitialize the destination before copy", and
+    // av_channel_layout_uninit() calls av_freep() on u.map — so a declaration
+    // without {} hands free() whatever pointer-shaped garbage the stack frame
+    // happened to hold. That is a real crash ("free(): invalid pointer"), not a
+    // theoretical one: it reproduced in roughly 1 run in 4 of UT-103, the only
+    // test that exercises this branch, because it is the only one whose input
+    // is stereo and so the only one that reaches the downmix path at all.
+    //
+    // It hid for two reasons worth remembering. It is stack-dependent, so it
+    // vanishes under a sanitizer build and looks like a flake in the aggregate
+    // test binary; and the golden-vector tests (UT-101) pass a mono 11025 Hz
+    // fixture, which is chosen precisely so the vector does not depend on the
+    // resampler — so bit-exactness against the golden vector proves nothing
+    // about this function.
+    AVChannelLayout out_layout{};
     av_channel_layout_default(&out_layout, 1);          // mono
-    AVChannelLayout in_layout;
+    AVChannelLayout in_layout{};
     if (av_channel_layout_copy(&in_layout, &f->ch_layout) < 0) return false;
     if (in_layout.nb_channels <= 0) {
         av_channel_layout_uninit(&in_layout);