diag(quic-interop): inspect surfaces [writer.* traces + histograms

After 'make build DEBUG=1 && run-matrix.sh' the writer emits per-drain
stats. inspect-multiplexing.sh now grep's them out of output.txt and
reports a stream_frames histogram + active-stream-count histogram, so
we can see at a glance whether the writer is iterating 64 active
streams but only emitting 1 (early-exit bug) or whether 'active=1'
because most streams were filtered out as isClosed (different bug).

If output.txt has no writer lines, the helper hints at how to enable
them.

quic/interop/inspect-multiplexing.sh

@@ -39,6 +39,24 @@ echo "=============== output.txt (runner stdout — last 200 lines) ============
 tail -n 200 "$CASE_DIR/output.txt" 2>/dev/null \
     || echo "(no output.txt)"
 
+echo
+echo "=============== writer-side debug traces (DEBUG=1 build only) ==============="
+# Per-drain frame/budget stats from buildApplicationPacket. Empty
+# unless the image was built with `make build DEBUG=1`.
+WRITER_LINES=$(grep -c '\[writer' "$CASE_DIR/output.txt" 2>/dev/null || echo 0)
+if [[ "$WRITER_LINES" -gt 0 ]]; then
+    echo "($WRITER_LINES writer trace lines; first 30:)"
+    grep '\[writer' "$CASE_DIR/output.txt" | head -n 30
+    echo
+    echo "stream_frames histogram (writer-reported):"
+    grep -oE 'stream_frames=[0-9]+' "$CASE_DIR/output.txt" | sort | uniq -c | sort -rn
+    echo
+    echo "active histogram (active stream count at drain time):"
+    grep -oE 'active=[0-9]+' "$CASE_DIR/output.txt" | sort | uniq -c | sort -rn
+else
+    echo "(no [writer.* lines — run with DEBUG=1 image: cd quic/interop && make build DEBUG=1)"
+fi
+
 echo
 echo "=============== server stderr (last 100 lines) ==============="
 tail -n 100 "$CASE_DIR/server/stderr.log" 2>/dev/null \