feat: add experimental OmniBuds sensor support

[TobiasRoeddiger]

OmniBuds is not currently recognized by the library. This adds an experimental integration derived from the public OmniBuds Android app 1.6.41, with both ears represented through their shared BLE connection.

• Expose per-ear accelerometer, gyro, magnetometer, green/red/IR PPG, object temperature, and four calculated vital-sign streams through the existing sensor/configuration interfaces.
• Decode batched samples and physical units while retaining raw values and packets. Serialize configuration writes, require acknowledgements, reject errors, and clean up on disconnect.
• Add single-ear/pair SVG illustrations, optional peer-status readback, and a protocol/rate/usage guide in doc/OMNIBUDS.md.

Draft pending OmniBuds hardware validation. Rates come from the APK, not throughput measurements. Its lowest accel/gyro setting is labelled 10 Hz but decoded as 12 Hz; the guide documents this discrepancy. No raw microphone contract was found, and named but unimplemented peripherals are not presented as verified typed sensors. Vital-sign reporting intervals and prerequisites still need hardware validation. No APK/decompiled vendor code is included.

Validation: Flutter 3.44.0; flutter analyze clean; all 68 library tests pass, including 19 OmniBuds tests with synthetic protocol fixtures; example widget test and web build pass. SVG pair preview inspected. Existing device code and package dependencies are unchanged apart from factory registration/exports and the asset directory.

[github-actions]

Visit the preview URL for this PR (updated for commit 0709912):

https://open-earable-lib-web-example--pr159-codex-omnibuds-sen-vuvhustz.web.app

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