feat(drivers): validate bounded IMU FIFO acquisition

Drain bounded MPU6000 and ICM-42688 FIFO batches. Publish the latest
validated sample only after overflow checks. Reject invalid drains.

Validation: driver, sensor and platform tests plus both ARM release builds
from a fresh build directory.

firmware/libs/kaptan-sensor/src/imu.rs

@@ -2,28 +2,21 @@
 
 use kaptan_time::Timestamp;
 
-/// Coherent IMU observation in the board body frame.
+/// Coherent IMU observation with explicit units.
 ///
 /// Acceleration is milligravity, angular rate is milliradians per second, and
-/// temperature is centidegrees Celsius. The board-frame transform is complete
-/// before this value is published.
-///
-/// Current MPU6000 and ICM-42688-P decoders round integer conversions toward
-/// zero. Across their configured ranges, decoded acceleration is within
-/// ±16,000 mg and angular rate within ±34,871 mrad/s, including mounting sign
-/// changes. Temperature is within -22,234..=27,233 cdegC. These are backend
-/// representation bounds over every raw i16 value, not physical operating
-/// limits or validation performed by this public snapshot type.
+/// temperature is centidegrees Celsius. Chip drivers return sensor-frame axes.
+/// The platform applies the board mounting transform before publication, so
+/// published observations use the body frame. This type does not validate values.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct ImuSnapshot {
-    /// Host monotonic acquisition-request time, captured immediately before the
-    /// accepted register read is submitted by the worker. This is neither DRDY
-    /// ISR entry nor the sensor's physical sampling epoch. Register updates,
-    /// filtering and deferred/coalesced interrupts prevent that inference.
+    /// Producer timestamp in the controller's monotonic clock domain.
+    /// The acquisition source defines its meaning. It need not represent the
+    /// sensor's physical sampling epoch.
     pub timestamp: Timestamp,
-    /// Body-frame acceleration in milligravity.
+    /// Acceleration in milligravity, in the producer's frame described above.
     pub acceleration_milligravity: [i16; 3],
-    /// Body-frame angular rate in milliradians per second.
+    /// Angular rate in milliradians per second, in the same frame as acceleration.
     pub angular_rate_milliradians_per_second: [i32; 3],
     /// Sensor temperature in centidegrees Celsius.
     pub temperature_centidegrees_celsius: i16,