Add ATR mavlink messages

[vlad-serbanica]

This PR adds an initial version of MAVLink messages for supporting ATR integration with AMC and other consumers.

Sequence example:

Drone processes frame F=12345, algorithm finds 3 targets. Drone emits, in order:

ATR_FRAME_INFO  { frame_id=12345, num_detections=3, frame_width=1920, frame_height=1080, sensor_id=0, ... }
ATR_DETECTION   { frame_id=12345, track_id=42, bbox=(810,420,120,80), class=MBT_1, confidence=87, status=ACTIVE }
ATR_DETECTION   { frame_id=12345, track_id=43, bbox=(1100,500,60,40), class=IFV_2, confidence=72, status=ACTIVE }
ATR_DETECTION   { frame_id=12345, track_id=44, bbox=(300,300,200,150), class=STRUCTURE, confidence=91, status=NEW }

Frame F=12346 — algorithm runs, finds nothing. Drone still emits:

ATR_FRAME_INFO { frame_id=12346, num_detections=0, ... }
That "empty header" is useful — it tells the consumer "I processed this frame, found nothing" vs. "you just didn't hear from me." Without it, silence is ambiguous.

Consumer-side state machine (the AMC video pipeline)

on receive ATR_FRAME_INFO(F, n, ...):
    state[F] = { expected: n, received: [], frame_meta: ... }

on receive ATR_DETECTION(F, ...):
    if F not in state: state[F] = { expected: ?, received: [], frame_meta: null }   # detection arrived first
    state[F].received.append(detection)

on video decoder produces frame F:
    if F in state and state[F].complete():
        render(frame, state[F].received)
    else:
        wait up-to-jitter-budget, then render with what we have
    drop state[F]

complete() is true when received.length == expected and frame_meta != null. The structure makes "we received the partial-burst" detectable without any timestamp arithmetic.