Sensor fusion: Euler↔quaternion conventions, calibration guidance, MMR mounting note

[lkasso]

Field-verified companions to the app's sensor-fusion polish (MetaWear-API-Swift PR #25), in the same spirit as the MMS NAND logging semantics section:

• Euler ↔ Quaternion Relationship (Field-Verified) — the firmware's Euler labeling vs its own quaternion: heading is classic ZYX yaw, but pitch is the negated classic roll and roll the negated classic pitch. Validated by paired captures of both channels at the same static poses (< 1° agreement on all three angles). Anyone post-processing quaternion logs into Euler needs this mapping.
• Euler register row — observed ranges (heading 0–360, pitch ±180, roll ±90) and that yaw duplicates heading sample-for-sample on fw 1.7.x.
• Calibration — per-sensor calibration motions (gyro: still; accel: rest on each face; mag: figure-8 away from metal), and realistic expectations: accuracy is a live trust score, the magnetometer gets demoted by field distortion constantly indoors, and medium (2) is the record-worthy threshold.
• MMR mounting note — the IMU's X/Y axes are rotated 90° about the face normal relative to the case; recommends tare-based rendering for case-relative visualization.

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