refactor(audio): extract DeviceSetup + shared rate-match helpers (phase 4)

Moves probeDeviceOptionsDual, applyDuplexSetup, applySplitSetup, and
teardownSplitMode verbatim into src/audio/engine/DeviceSetup.{h,cpp}. The
component holds references to the two device managers + EngineState and owns
no lifetime; engine-owned collaborators (monitor chain, split output ring +
counters, output callback registration) are passed by reference per call.
setAudioDevices stays on the facade as the orchestrator. The public
DeviceOptions/DeviceConfig/DeviceConfigResult shapes move to the slopsmith
namespace with using-aliases on AudioEngine, so the NodeAddon spelling is
unchanged.

Lands the deep-read §7 dedupe structurally: the <=0.5 rate tolerance,
midpoint-rounding fail-closed candidate, and empty-name→first-enumerated
resolution now exist once (RateMatch.h — JUCE-free + unit-tested boundary
cases — and DeviceSetup::resolveDeviceName/rateSupportedBy) instead of three
hand-synced copies.

Full device-matrix validation (WASAPI shared/exclusive, ASIO, dual-type
split) rides the next tester build per the plan's phase-4 gate.

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

tests/engine_units/CMakeLists.txt

@@ -20,3 +20,7 @@ add_test(NAME engine_state COMMAND engine_state_test)
 add_executable(renderer_bus_test renderer_bus_test.cpp)
 target_compile_features(renderer_bus_test PRIVATE cxx_std_20)
 add_test(NAME renderer_bus COMMAND renderer_bus_test)
+
+add_executable(rate_match_test rate_match_test.cpp)
+target_compile_features(rate_match_test PRIVATE cxx_std_17)
+add_test(NAME rate_match COMMAND rate_match_test)