docs: the TCPStreamer source kind, the client's TCP mode, and its probes

README.md

@@ -327,6 +327,27 @@ delivers one frame per datagram. The source is connected on the first valid data
 was verified on a loopback group (`239.255.42.99` joined on `127.0.0.1`, although this machine's `lo` does not
 carry the `MULTICAST` flag); a real network, its switches and IGMP were not exercised.
 
+The fourth built-in kind is **TCPStreamer** (`--source tcps`), a receiver for the MARTe2 `TCPStreamer`
+datasource, standalone or through a shared `TCPStreamService`. Both speak the UDPStreamer framing over a TCP
+connection, so it is the UDPS client in a TCP mode: the same CONFIG and DATA, the same channels, calibration and
+timing. The form takes the server (default 127.0.0.1), the port (44500) and an optional **stream name**: leave
+it empty for a standalone `TCPStreamer`, which starts sending on connect; name the stream for a
+`TCPStreamService`, which wants a `SUBSCRIBE` first (the client sends one only when a name is set). On the
+command line these are `--source tcps --source-opt server=... --source-opt port=... --source-opt stream=...`.
+The source is disconnected after 2 s of silence or a closed connection and reconnects every 0.5 s; a stream
+whose frames are more than 2 s apart therefore reconnects continually, as UDPS does. It is verified against
+`tools/mock_tcpstreamer.py`, not against a MARTe2 application. Its limits:
+
+- **One stream per connection.** A multiplexed `SUBSCRIBE` list (`a,b,c`) is refused by the form, because a
+  source is single-timebase; merge streams on the server with `TCPStreamJoin` and name the result.
+- **No packet capture.** The capture is of the UDP socket; the button stays disabled.
+- **A gap is reported at once.** TCP is ordered, so there is no reorder window: a counter gap is a server-side
+  drop and is counted as lost immediately.
+- **A frame is at most `UDPS_MAX_DATAGRAM_BYTES` (about 65 KB)**, which the server's `MaxPayloadSize` default of
+  60000 satisfies.
+- **After a reconnect to a server whose time base restarted** the app prints `the clock stalled at ...`
+  warnings, exactly as for a restarted UDPS stream.
+
 A session file written before kinds existed has no `source_kind`, and is read as UDPS; a UDPS file is
 written exactly as before. A kind's capability panels (for instance a source's own controls) appear as
 pages of the side stack, and are insensitive rather than hidden while no source is connected.
@@ -3299,6 +3320,10 @@ silence timeout and then resumes, which is the EMI case end to end.
   exactly faithful in `strict` mode, and the only reading of the wire that does
   not invent samples in `accumulate` mode. The info popup and the CONFIG dump on
   stderr report the element counts.
+- **The TCPStreamer source takes one stream per connection and has no packet capture.** A multiplexed
+  `SUBSCRIBE` list is not supported (merge on the server with `TCPStreamJoin`), the capture button stays
+  disabled, a gap is reported at once (no reorder window) and a frame is limited to about 65 KB; see "Data
+  sources".
 - **The event window can be shorter than the buffer.** The trigger latches a window
   of the event-span selection; the ring holds far more, so with trigger mode off the
   rest is scrollable history.