perf: reduce 5A chunk timeout to 10s and stop iteration at requested event index

Two improvements to eliminate the ~2-min-per-event wait and unnecessary
full-event-list download when only one event is requested:

1. protocol.py: pass timeout=10.0 to _recv_one in the 5A chunk loop.
   Device responds within ~1s per chunk; 10s gives a safe 10x buffer.
   End-of-stream detection (raw_bytes=1) now fires in 10s instead of 120s,
   cutting ~110s of dead wait per event.

2. client.py: add stop_after_index parameter to get_events(). When set,
   iteration stops immediately after the target event is collected — no
   further 0A/1E/0C/5A/1F cycles for events the caller doesn't need.

3. server.py: pass stop_after_index=index to both /device/event/{idx}
   and /device/event/{idx}/waveform endpoints so a single-event request
   only downloads that one event.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sfm/server.py

@@ -380,7 +380,7 @@ def device_event(
         def _do():
             with _build_client(port, baud, host, tcp_port) as client:
                 client.connect()
-                return client.get_events()
+                return client.get_events(stop_after_index=index)
         events = _run_with_retry(_do, is_tcp=_is_tcp(host))
     except HTTPException:
         raise
@@ -433,9 +433,8 @@ def device_event_waveform(
         def _do():
             with _build_client(port, baud, host, tcp_port, timeout=120.0) as client:
                 info = client.connect()
-                # full_waveform=True fetches the complete 5A stream inside the
-                # 1E→0A→0C→5A→1F loop.  Issuing a second 5A after 1F times out.
-                events = client.get_events(full_waveform=True)
+                # stop_after_index avoids downloading events beyond the one requested.
+                events = client.get_events(full_waveform=True, stop_after_index=index)
                 matching = [ev for ev in events if ev.index == index]
                 return matching[0] if matching else None, info
         ev, info = _run_with_retry(_do, is_tcp=_is_tcp(host))