refactor(audio): extract PackedStereoRing — one SPSC ring template (phase 1)

Replaces the four hand-maintained copies of the packed-LR SPSC design
(split-mode output ring, per-InputDeviceSlot rings, stream-sink ring,
renderer-bus ring) with slopsmith::PackedStereoRing<NFrames>
(src/audio/engine/PackedStereoRing.h). The template owns the storage,
power-of-two/lock-free asserts, pack/unpack, producer publish, reset, the
w<r resync, and the lapped catch-up; per-site consumer policy (pull-vs-
consume skew, renderer prime/fill-clamp) stays verbatim at the call sites.

Pure code move per the TLC plan — no behavior change; the renderer bus's
control-thread readIndex write on disable (deep-read §4) is deliberately
preserved and gets its flush-flag fix in phase 2.

Unit-tested in tests/engine_units/packed_stereo_ring_test.cpp: threaded
tear-freedom under lapping (2M frames), drop-oldest catch-up, index-reset
resync, pull-vs-consume skew.

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

tests/engine_units/CMakeLists.txt

@@ -5,3 +5,10 @@
 add_executable(gain_sanitize_test gain_sanitize_test.cpp)
 target_compile_features(gain_sanitize_test PRIVATE cxx_std_17)
 add_test(NAME gain_sanitize COMMAND gain_sanitize_test)
+
+# PackedStereoRing uses std::bit_cast (C++20) and std::thread.
+add_executable(packed_stereo_ring_test packed_stereo_ring_test.cpp)
+target_compile_features(packed_stereo_ring_test PRIVATE cxx_std_20)
+find_package(Threads REQUIRED)
+target_link_libraries(packed_stereo_ring_test PRIVATE Threads::Threads)
+add_test(NAME packed_stereo_ring COMMAND packed_stereo_ring_test)