Burst acquisition: many whole rows per FastFrame acquisition

Per-row acquisition pays a full arm/stop/transfer round trip for every row,
and the transfer is one IEEE-488.2 block read per frame (~16k frames a row).
Burst mode runs one FastFrame acquisition across as many complete rows as the
scope's frame memory holds and pulls each burst in a single CURVe?
transaction, amortising the round trip over the whole burst.

It is opt-in (ScanEngine(burst_mode=...), default False) and writes
byte-identical files to the per-row path — test_burst_and_serial_produce_
identical_files runs the same plan both ways and compares the bytes, which is
the property the whole feature rests on.

core/scope_burst.py — the new policy module. Everything that computes rather
than talks to hardware is a free function, so sizing and row-splitting are
testable without a rig: rows_per_burst() (rounds down, since a partial row
can't be written, and clamps to a transfer-buffer budget), split_row_counts(),
normalize_row(), frame_means_block().

The hard part is that a burst carries no row markers — the scope returns one
flat run of frames. Boundaries come from ACQuire:NUMFRAMESACQuired? sampled
after each acquiring pass while the stage gate is already low, rebased on a
baseline read back at RUN rather than assuming the counter resets. A counter
that goes backwards means the acquisition restarted mid-burst and is now a
hard error instead of silently misattributing every later row.

core/scan_engine.py — the row loop splits into _scan_rows_serial and
_scan_rows_burst. The wire is channel-major and the file is row-major with
channels inner, so _write_burst deinterleaves by writing one channel at a
time to strided offsets; peak memory stays at a single channel's burst
instead of the whole thing.

_gate_off_preflight is what makes this trustworthy on real hardware. The BBD
value that idles the trigger output low is not settled by the protocol docs
(see TRIGOUT_GATE_OFF), and getting it wrong fills every burst with flyback
frames that silently shift the file. The scope already measures the gate on
CH3, so the check needs no bench probe: one gated-off flyback must acquire
nothing, and one gated pass must acquire something — the second half is what
stops a dark laser from making the first half pass vacuously. It runs once
per scan and costs two row-times.

Two fixes fall out of this work and apply to both paths:
- Rows are now squared up to the declared n_frames (short rows zero-padded,
  long rows truncated, both warned). v6 commits to n_frames per row in the
  header and has no per-row length field, so an over- or under-triggered row
  used to shift every later row in the file.
- The X trigger output is returned to idle in the run() finally block. The
  per-row path left TRIGOUT_MAXV armed for the rest of the session, so the
  gate line kept being driven on every later jog.

core/scope_sras.py — pins DATa:ENCdg RIBinary and DATa:WIDth 1 during setup
instead of inheriting front-panel state. The file header hardcodes
bytes_per_sample=1; a scope left on 2 bytes would have corrupted every frame
written. frames_acquired/frame_means move to scope_burst, where the offset-
based variants serve both paths.

tests/fakes.py — FakeStage and FakeScope are now wired together the way the
rig is: a gated X move at scan velocity feeds frames into a running
acquisition at the real 20 kHz / 100 mm/s rate, direction-agnostic. Both
paths therefore derive frame counts from one model, which is what makes the
byte-identity comparison meaningful, and a gate the engine forgets to drop
shows up as extra frames instead of passing silently. Frame content is a
function of (channel, index) alone, so the same frame sequence yields the
same bytes however it is chopped into transfers.

87 tests passing, ruff clean.

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

core/scope_sras.py

@@ -10,8 +10,6 @@ import logging
 import time
 from dataclasses import dataclass
 
-import numpy as np
-
 logger = logging.getLogger(__name__)
 
 SAMPLE_RATE_HZ = 6.25e9    # 6.25 GS/s → 160 ps/sample
@@ -66,6 +64,11 @@ def configure_acquisition(scope) -> int:
     scope.set_trigger_mode("NORMAL")       # wait for trigger (don't auto-sweep)
     scope.set_acquire_mode("SAMPLE")
     scope.set_fastframe_state(False)
+    # Pin the transfer format instead of inheriting front-panel state — the
+    # file header hardcodes bytes_per_sample=1, and a scope left on 2 bytes
+    # would corrupt every frame written.
+    scope.set_data_encoding("RIBinary")
+    scope.set_data_width(1)
     scope.set_sample_rate(SAMPLE_RATE_HZ)
     scope.write("HORizontal:POSition 30")  # 10 % trigger offset
     time.sleep(0.3)                        # let the timebase settle before reading back
@@ -140,28 +143,7 @@ def finish_row(scope) -> None:
     scope.write("ACQuire:STATE STOP")
 
 
-def frames_acquired(scope) -> int:
-    return int(scope.query("ACQuire:NUMFRAMESACQuired?"))
-
-
 def transfer_channel(scope, ch: int) -> list[bytes]:
     """Fetch one channel's FastFrame block as raw int8 frames."""
     scope.set_data_source(ch)
     return scope.transfer_fastframe(parse=False)
-
-
-def frame_means(waveforms: list[bytes]) -> list[float]:
-    """Per-frame DC mean of a raw int8 FastFrame block.
-
-    numpy over the joined buffer: the per-frame struct.unpack this replaces
-    allocated a tuple of Python ints per frame (~16k frames per row).
-    """
-    if not waveforms:
-        return []
-    n = len(waveforms[0])
-    if n == 0 or any(len(w) != n for w in waveforms):
-        # Ragged block (shouldn't happen) — fall back to per-frame means.
-        return [float(np.frombuffer(w, dtype=np.int8).mean()) if len(w) else 0.0
-                for w in waveforms]
-    block = np.frombuffer(b"".join(waveforms), dtype=np.int8).reshape(len(waveforms), n)
-    return block.mean(axis=1, dtype=np.float32).tolist()