Decode compact PPG and support firmware 2.3.x

[TobiasRoeddiger]

Decode firmware 2.3.x BLE PPG as four lossless 19-bit values packed into ten bytes. Each device factory reads the firmware version and creates its own decoder, so 2.2.9/2.2.10 and mixed-version pairs retain the legacy path. Extend the supported firmware range to >=2.1.0 <2.4.0.

The existing sensor scheme still supplies logical channel metadata. The default/file parser remains legacy; SD/.oe decoding and other sensor payloads are unchanged. No packet marker or sensor-ID change is used.

Depends on protocol #1. Publish open_earable_protocols 0.0.4 before releasing this library; the temporary root/example overrides pin the reviewed protocol commit so this PR can be tested beforehand. Apps testing this unpublished dependency must also use that override.

Validation: all 18 library tests pass; all seven compatibility/PPG tests also pass on Chrome; full library analysis passes. Tests cover firmware/prerelease selection, equal-length old/new packets, 2,300 randomized samples, full-scale channels, timestamp reconstruction, ByteData views, malformed packets, and unchanged legacy/non-PPG decoding.

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