Steam Controller: switch IMU to raw gyro/accel — all 3 axes working

[petegordon]

Follow-up to #11. Pivots the Steam Controller's IMU pipeline from the quaternion-output path that #11 shipped to the standard rate-based DualSense path, and all 3 axes now work correctly.

What changed

Per-axis Test Report variance analysis revealed the actual 2026 firmware IMU layout differs from what SteamlessController documents:

bytes 29-32 = uint32 LE timestamp (~1 MHz clock)
bytes 33-38 = 3-axis accelerometer (int16 LE, ±2g full scale)
bytes 39-44 = 3-axis gyroscope    (int16 LE, ±2000 dps)
bytes 45+   = always-zero padding

The numbers at rest match a DualSense exactly — gyro is 0 ± 0.2 dps zero-bias across all axes, and accel reads ~1g of gravity on body-Z. So they flow straight into the existing SensorFusion pipeline (calibration, drift correction, L3+R3 recalibrate, integrated orientation) with zero special-casing.

Driver-side remap

Body-to-visualizer frame: swap Y↔Z and negate Z on both gyro and accel. The swap moves gravity from body-Z to body-Y (Three.js "Y up" convention, matches DualSense). The negation aligns roll direction. Same pattern Switch Pro uses for its frame remap.

App.js cleanup

Removes everything from #11 that no longer applies:

• _steamRawQuat / _steamDelta / _steamRefQuat state
• recalibrateSteamReference + setSteamQuatTransform DevTools API
• 16-mode STEAM_QUAT_TRANSFORMS registry
• parsed.orientation fast-path
• Calibration-skip guard (we DO calibrate now, same as DualSense)

Roughly -106 lines from app.js, +30 driver lines (mostly comments explaining the remap), +5 profile lines.

Field verification

Physical pitch / roll / yaw all rotate the on-screen model on the correct axis with correct sign. Calibration completes in ~1 second. L3+R3 recalibrates.

Known minor issue

~6-7° residual pitch drift settling on initial calibration — accel-based gravity correction nudging the orientation to match the measured gravity vector. Acceptable for now; can be tuned later via SensorFusion calibration thresholds.

Commit

• 28ef7f6 switch IMU to raw gyro/accel — all 3 axes work

Test plan

• Pitch the controller → model pitches on the pitch axis
• Roll the controller → model rolls on the roll axis (correct sign)
• Yaw the controller → model yaws on the yaw axis (yaw was broken in Steam Controller: Phase 3a — IMU quaternion to visualizer + Test Report multi-interface fix #11)
• Calibration completes ~1s on connect
• L3+R3 recalibrates
• No quaternion-related DevTools spam or stale state

Refs #8.