Use Float32 for RX capture path

[mrmidi]

Summary

• Advertise RX/input physical stream format as native Float32 to match the already-working TX/output path.
• Convert the direct RX capture binding and writer path to write float* input buffers.
• Keep AM824/raw handling at the wire edge, including synthesizing the 0x40 MBLA label for Saffire raw RX slots before decoding.
• Update direct-audio runtime tests and add RX packet processor coverage for AM824 Float32 capture and raw Saffire label normalization.

Validation

• ./build.sh --test-only passed: 1089/1089 run tests.
• ctest --test-dir build/tests_build -V -R IsochRxTiming passed.
• git diff --check clean.
• Hardware verified on Focusrite Saffire: TX and RX both working; Audio MIDI shows input/output physical and virtual formats as Float32.