perf(micromate): 16 KB per request, not 1024 -- 14x fewer round trips

The chunk-size ceiling, measured on UM20147 with every size checked
byte-for-byte against a known-good download:

  1024 / 2048 / 4096 / 8192 / 16384  ->  served in full
  32768 / 65535                      ->  SILENTLY CLAMPED to 16384

The clamp is the important part: a 32,768 B request returns 16,384 B of perfectly
good data and no error.  Nothing in the response says it was truncated; the
length is the only signal.

THAT DICTATES HOW THE LOOP MUST BE WRITTEN.  read_event_file() now tracks its
offset by BYTES RECEIVED rather than striding by chunk index.  A fixed stride
would either fail on a clamp or skip the bytes it never collected; tracking what
actually arrived makes a clamp cost one extra request, and makes the loop
self-correcting against any short response -- precisely the failure mode that has
bitten the Series III side repeatedly.

Consequence of that change, recorded because it reverses an earlier decision: a
short chunk is NO LONGER AN ERROR.  It used to raise, on the principle that a
silently short event is this codebase's recurring bug.  But the device returns
short legitimately, and the real protection is the offset arithmetic plus the
final total-length check -- which still raises on a genuinely truncated event.
The test was rewritten rather than deleted, and says why.

CHUNK_SIZE = 16384.  For UM20147's events: 4,796 B goes 5 requests -> 1,
30,230 B goes 30 -> 2, and 72,560 B goes 71 -> 5.  Over cellular at ~0.65 s per
round trip that is ~46 s -> ~3.2 s on the large one.

Measured on one unit over USB, so THOR_CHUNK_SIZE = 1024 stays available and the
replay tests pin it -- reproducing THOR's exact traffic is one argument away, and
client.download_event()/get_event() take chunk_size for a link where large
responses are not surviving.

Full suite: 472 passed, 16 pre-existing failures unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ru8Lg9HkkYvX9VWWo65SmL

tests/test_micromate_events.py

@@ -293,7 +293,9 @@ def test_replaying_thors_session_reproduces_every_byte_and_decodes_every_event()
 
     decoded, checked = 0, 0
     for ref in mm.iter_events():
-        result = mm.get_event(ref)          # verify=True by default
+        # Pin THOR's chunk size: this test asserts byte-identity with
+        # THOR's frames, and our default is 16x larger.
+        result = mm.get_event(ref, chunk_size=1024)
         decoded += 1
         assert ref.serial == "UM12947"
         assert ref.filename and ref.filename.endswith(ref.suffix)
@@ -334,10 +336,10 @@ def test_a_corrupted_stored_peak_is_caught_by_the_self_check():
 
     for ref in mm.iter_events():
         if ref.is_histogram:
-            mm.download_event(ref)          # keep the replay in step
+            mm.download_event(ref, chunk_size=1024)   # keep the replay in step
             continue
         ref.peak_vector_sum_ips *= 1.5      # as a bad decode would look
         with pytest.raises(DecodeMismatch, match="decode is suspect"):
-            mm.get_event(ref)
+            mm.get_event(ref, chunk_size=1024)
         return
     pytest.fail("no waveform event found in the capture")