preserve transport outage validation evidence

README.md

@@ -66,12 +66,19 @@ overruns. Hardware data-ready interrupts and FIFO acquisition are deferred to th
 later sensor-side acquisition refinement.
 
 Completed samples enter a 512-record RAM queue, providing 5.12 seconds of
-blocked or disconnected transport tolerance at 100 Hz. A failed write retains
-and retries its packet while this queue accumulates the backlog. A lower-priority
-output task batches up to eight records into versioned `TRK1` frames, isolating
-acquisition from brief USB or future BLE stalls. CRC, packet and sample sequences,
-timestamps, and cumulative loss/overrun counters make any queue overflow
-detectable.
+transport-outage tolerance at 100 Hz when the transport reports backpressure or
+failure accurately. A failed write retains and retries its packet while this
+queue accumulates the backlog. A lower-priority output task batches up to eight
+records into versioned `TRK1` frames, isolating acquisition from brief transport
+stalls. CRC, packet and sample sequences, timestamps, and cumulative
+loss/overrun counters make permanent loss detectable by the receiver.
+
+The current ESP-IDF USB VFS path reports physical disconnects, but a connected
+host that stops draining can time out below stdio and still appear successful to
+firmware. The host can detect resulting loss from packet/sample sequences and
+CRC framing, but the device cannot count that case. A direct USB driver with
+bounded drain waits, and ultimately receiver acknowledgements with replay, are
+deferred to the common USB/BLE transport layer.
 
 Measured end-to-end framing overhead is about 2.47 kB/s at 100 Hz, or 8.47
 MiB/hour before BLE link overhead.