add xArm driver layer ported from littlehand

Brings across the robot layer from monolitic-littlehand-python, which was
written against this same controller. The point of it is that a command
either moved the arm or raised: the SDK returns a code for a motion the
controller quietly refused, wait_move calls that refusal finished, and
arm.error_code is the report stream's stale copy.

So errors are read live, arrival is confirmed rather than assumed, and a
move whose straight-line path leaves the joint envelope is refused before
it is commanded. Failures are typed, because the distinctions decide the
response: an unreachable pose is not an unavailable arm, and a joint past
its limit needs a motion nobody should start without warning the room.

read_report is the deliberate exception to all of it: a report-cache read
that costs no round trip and cannot block, which is the only read that
works while a motion is in flight.

dora_ulite6/log.py

@@ -0,0 +1,13 @@
+"""Timestamped stderr logging.
+
+stderr rather than stdout: dora reads a dynamic node's stdout, and a print that
+lands there is noise in the wrong stream.
+"""
+
+import sys
+import time
+
+
+def log(message: str) -> None:
+    """Print a timestamped line for the node."""
+    print(f"[ulite6 {time.strftime('%H:%M:%S')}] {message}", file=sys.stderr, flush=True)