Driver support for multi-row FastFrame bursts; fix read_raw block parsing

Groundwork for burst acquisition: the scope needs to report and transfer a
whole multi-row FastFrame acquisition, and the BBD needs to gate its trigger
output per row rather than staying armed for the scan.

tektronix_base
- get_fastframe_max_frames() exposes HORizontal:FASTframe:MAXFRames?, which
  is what sizes a burst once the horizontal settings are fixed.
- transfer_fastframe_bulk() pulls a burst as one contiguous buffer. Unlike
  transfer_fastframe it does not assume how the scope frames the response:
  it accumulates until the expected byte count is reached, so one large IEEE
  block and one block per frame both work.
- set_data_encoding() / set_data_width() make the transfer format settable
  instead of inherited from whatever the front panel was left on.
- read_raw() had two real defects. The length-digit read used a bare recv()
  and only checked the length afterwards, so a short read raised "Failed to
  read data length" on a perfectly good transfer; it now goes through a
  _recv_exact() helper, as does the trailing separator. And a #0
  indeterminate-length block was parsed as int("") -> ValueError. #0 is
  normally delimited by EOI, which a raw socket never sees, so read_raw now
  takes expected_bytes to size it. The bulk transfer relies on this.

pybbd202
- arm_scan_gate(axis, armed) raises and drops the max-velocity trigger
  output the scope's AND-gate uses. A burst spans several rows with the
  scope running throughout, so the gate must be low for the flyback or the
  return move reaches max velocity and injects frames between rows.
- set_trigger_verified() reads the mode back after setting it. set_trigger
  is fire-and-forget over the shared TX queue; burst mode toggles the gate
  between every row, where a dropped change silently corrupts the file
  rather than failing loudly.
- set_trigger_gate_off() so the scan can leave the output idle on exit.
- TRIGOUT_GATE_OFF is deliberately marked unverified. §7.6 of the BBD203
  protocol doc describes `mode` as an enumeration capping at 0x11, which
  contradicts the bitmask this driver actually sends (TRIGOUT_MAXV = 0x90,
  known working), so the doc cannot settle which value idles the pin low.
  The engine's preflight check resolves it on the rig instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

hardware/pybbd202/bbd20x.py

@@ -7,7 +7,7 @@
 import time
 from threading import Thread, Event
 from queue import Queue, Empty
-from .apt_constants import StatusBits, TriggerBitsServo
+from .apt_constants import StatusBits, TriggerBitsServo, TRIGOUT_GATE_OFF
 from .apt_messages import APTProtocol
 from .serial_comms import SerialSnooper
 
@@ -465,3 +465,42 @@ class ThorlabsServoDriver():
     def set_trigger_trigout_maxv(self, axis):
         '''Set trigger output high + pulse at max velocity (TRIGOUT_MAXV).'''
         self.set_trigger(axis, TriggerBitsServo.TRIGOUT_MAXV)
+
+    def set_trigger_gate_off(self, axis):
+        '''Drive the trigger output inactive, so no pulses reach the gate.'''
+        self.set_trigger(axis, TRIGOUT_GATE_OFF)
+
+    def arm_scan_gate(self, axis, armed, verify=True):
+        '''
+            arm_scan_gate(axis, armed): Arms or drops the max-velocity trigger
+            output the oscilloscope AND-gate uses.
+
+            Burst acquisition runs one scope acquisition across many rows, so
+            the gate must be armed only for the acquiring pass and dropped for
+            the flyback — otherwise the return move hits max velocity and
+            injects frames between rows.
+        '''
+        mode = TriggerBitsServo.TRIGOUT_MAXV if armed else TRIGOUT_GATE_OFF
+        if verify:
+            self.set_trigger_verified(axis, mode)
+        else:
+            self.set_trigger(axis, mode)
+
+    def set_trigger_verified(self, axis, mode, timeout=5.0, retries=2):
+        '''
+            set_trigger_verified(axis, mode): Sets the trigger mode and reads
+            it back to confirm it landed.
+
+            set_trigger is fire-and-forget over the shared TX queue.  Burst
+            acquisition toggles the gate between every row, and a dropped
+            change there silently fills the acquisition with flyback frames —
+            so confirm rather than assume.
+        '''
+        for _ in range(retries + 1):
+            self.set_trigger(axis, mode)
+            if int(self.get_trigger(axis, timeout=timeout)) == int(mode):
+                return
+        raise RuntimeError(
+            f"Axis 0x{axis:02X} did not accept trigger mode 0x{int(mode):02X} "
+            f"after {retries + 1} attempts"
+        )