docs: the source-plugin framework, the --source options and the source-kinds probe

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

README.md

@@ -27,6 +27,7 @@ Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb`
 window the toolbar offers, default 600), `--holdoff` (default trigger hold-off),
 `--recv-buffer`, `--plots` (the number of stacked plots to start with, 1..8; rows are
 added and removed from the **Layout editor** afterwards), `--relative-time` (see below),
+`--source NAME` and the repeatable `--source-opt k=v` (the source kind and its settings; see "Data sources" below),
 `--capture-packets N` (see "Packet capture" below), `--renderer NAME` (GTK's renderer:
 `cairo`, `gl` or `vulkan`; `cairo` is the software preset that exports both
 `GSK_RENDERER` and `LIBGL_ALWAYS_SOFTWARE` before GTK initialises — see the SSH
@@ -98,13 +99,30 @@ src/event_series.hpp  an immutable captured event as a plot::data::Series
 src/ui.*           the GTK4 window: toolbar, trigger panel, signal panel, plot box
 ```
 
-Threading: one producer thread owns the `udps_client_t` and calls
-`udps_client_poll()`; the GTK main thread owns every widget and the GL contexts.
+Threading: one producer thread owns the data source (the UDPStreamer client is the built-in
+one) and calls `poll()`; the GTK main thread owns every widget and the GL contexts.
 They share `ScopeModel` under one mutex, which the producer takes once per DATA
 frame and the GUI thread takes once per frame around binding the traces.
 Samples cross the boundary through `plot::data::RingBuffer` (single producer,
 single consumer), never through the lock.
 
+## Data sources
+
+What udpscope shows is fed by a *source kind*; the connection chooser's **Source type** row picks one and
+swaps in that kind's own connection form. The built-in kind is **UDPS**, the UDPStreamer client this
+document describes everywhere else, and it is the default: `--server`, `--port`, `--multicast` and
+`--interface` populate its entries, and the same thing is spelled `--source udps --source-opt server=...`.
+`--help` lists the registered kinds. A second kind, `probe` (a synthetic sine with one panel), is registered
+only when `UDPSCOPE_PROBE_SOURCE_KINDS` is set, and exists only for the self-check.
+
+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.
+
+Planned, each as its own sub-project: a flat UDP kind, the MCC E-1608 and ABScope. ABScope has more than one
+timebase, so it will change the frame and the data path together rather than fit the present single-timebase
+frame.
+
 ## The x axis is relative to now
 
 **A live axis is fixed at `[-window, 0]`, with the newest sample at 0.** That is what a bench