feat(audio): bind the v1 signature and validate offset recovery on real content

sae_audio exposes the shipped signature to Python. It compiles
audio_signature.cpp directly against FFmpeg rather than linking
sae_gallery: the signature needs no model, no OpenCV and no HDF5, so a
module that dragged those in would make `import sae_audio` depend on a
GPU-capable build of a path that is pure CPU DSP.

The point of binding rather than porting is that a fingerprint is only
useful if every implementation agrees byte for byte. A numpy port would
be a third implementation, and the one nobody checks against the golden
vector.

VR-014 then recovers a known trim from real film audio rather than from
the synthetic tone: 40 random in-cap offsets, every one recovered to the
nearest frame, worst error 46 ms against a 500 ms budget — and 46 ms is
the quantisation floor, not a result, since offsets land on whole
92.88 ms frames.

The soft spot is tier labelling rather than accuracy. Sub-frame
misalignment drags the score down (0.94-0.99 near a frame boundary,
0.69-0.73 at half a frame), demoting 27 of 40 correct alignments to
`loose`. Allowing +/-1 frame of slack in the score fixes it: all 40 back
to `audio` at min 0.906, false matches unmoved at 0.12-0.16, for 81 ms
of the budget.

The module stops at the producer's edge. Sliding one signature against
another is the consumer's algorithm (server SPEC §3, and the jRay
plugin implements it), so a caller writing that slide in numpy is not
duplicating anything this repo owns.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: IR-004, IR-005 | VR-014 | UT-105, UT-106, UT-107, UT-108 | SR-003

CMakeLists.txt

@@ -308,6 +308,16 @@ target_link_libraries(sae_embed PRIVATE sae_gallery)
 nanobind_add_module(sae_kpn src/kpn_bindings.cpp)
 target_link_libraries(sae_kpn PRIVATE sae_gallery)
 
+# ── sae_audio — Python module: the v1 audio signature (IR-004) ────────────────
+# Compiles audio_signature.cpp directly and links only FFmpeg, rather than
+# linking sae_gallery: the signature needs no model, no OpenCV and no HDF5, and
+# a module that dragged all three in would make `import sae_audio` depend on a
+# GPU-capable build of a repo whose audio path is pure CPU DSP. tests/ compiles
+# the same source the same way, for the same reason.
+nanobind_add_module(sae_audio src/audio_bindings.cpp src/audio_signature.cpp)
+target_include_directories(sae_audio PRIVATE src)
+target_link_libraries(sae_audio PRIVATE ffmpeg_libs)
+
 # HDF5 already found above (before sae_gallery); vars HDF5_CXX_LIBRARIES / _INCLUDE_DIRS
 # are reused by scene_analyze / dump_embeddings below.