Add IMU data logger to SD card with remote start/stop

Streams BHI260AP accelerometer (50Hz), gyroscope (50Hz), and
magnetometer (25Hz) as timestamped CSV to SD card. Ring buffer
in PSRAM (512 samples) flushed to SD from the main loop.

Remote debug command 'L' toggles logging. Python tool:
  ./scripts/screenshot.py log

CSV format: ms,ax,ay,az,gx,gy,gz,mx,my,mz (raw int16 units)
Measured throughput: ~43Hz actual sample rate.

Also fixed: BHI260AP init no longer gated on hw_ready (radio
provisioning) — IMU sensors should work regardless of radio state.

Foundation for compass calibration (PCA on magnetometer data),
gesture recognition training, and activity classification.

scripts/screenshot.py

@@ -158,6 +158,24 @@ def cmd_invalidate(s):
     print("Invalidated — full redraw requested")
 
 
+def cmd_log(s):
+    send_cmd(s, ord('L'))
+    buf = b""
+    deadline = time.time() + 5
+    while time.time() < deadline:
+        chunk = s.read(max(1, s.in_waiting or 1))
+        if chunk:
+            buf += chunk
+        magic = PREFIX + b"L"
+        idx = buf.find(magic)
+        if idx >= 0:
+            nl = buf.find(b"\n", idx + 4)
+            if nl >= 0:
+                print(buf[idx + 4:nl].decode())
+                return
+    print(f"Timeout ({len(buf)} bytes)")
+
+
 def main():
     parser = argparse.ArgumentParser(description="R-Watch remote debug")
     parser.add_argument("-p", "--port", default=DEFAULT_PORT)
@@ -181,6 +199,9 @@ def main():
 
     sub.add_parser("invalidate", aliases=["inv"])
 
+    sub.add_parser("log", aliases=["l"],
+                    help="Toggle IMU logging to SD card")
+
     args = parser.parse_args()
     if not args.command:
         parser.print_help()
@@ -200,6 +221,8 @@ def main():
             cmd_navigate(s, args.screen)
         elif args.command in ("invalidate", "inv"):
             cmd_invalidate(s)
+        elif args.command in ("log", "l"):
+            cmd_log(s)
     finally:
         s.close()