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-drivers/src/mpu6000/test_bus.rs

@@ -11,27 +11,27 @@ pub(crate) enum FakeError {
 }
 
 pub(crate) struct FakeBus {
-    pub response: [u8; 15],
+    pub response: [u8; 32],
     pub fail_start: bool,
     pub fail_completion: Option<CompletionError>,
     pub busy: bool,
     pub starts: usize,
     pub cancellations: usize,
-    pub transmitted: [u8; 15],
-    pending: Option<([u8; 15], usize)>,
+    pub transmitted: [u8; 32],
+    pending: Option<([u8; 32], usize)>,
     quarantined: bool,
 }
 
 impl FakeBus {
     pub fn ready() -> Self {
         Self {
-            response: [0; 15],
+            response: [0; 32],
             fail_start: false,
             fail_completion: None,
             busy: false,
             starts: 0,
             cancellations: 0,
-            transmitted: [0; 15],
+            transmitted: [0; 32],
             pending: None,
             quarantined: false,
         }