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>
