document the source chooser

Both files, per this project's rule: the probe entries and the two invariants
in CLAUDE.md, the feature and why the list is filtered in README.

README.md

@@ -458,6 +458,30 @@ a math channel's name and that name only exists as a signal once its graph is in
 reopened on the same source comes back as the one that was closed — including a signal that was
 enabled only to feed a math channel.
 
+**Launched with no `--server`, udpscope asks which source to open.** It used to connect to
+`127.0.0.1:8000` because that is the default in `Config` — fine for one streamer, useless for
+several, and there was no way to see which sources the app already knew about. The chooser lists
+one row per stored session: the server and port it was named after, how many plots and signals it
+held, and its multicast group when it had one, newest first. `New host…` is a four-field form —
+server, port, multicast group, interface — and nothing more: `--buffer-mb` and the rest are tuning
+rather than "which source", and a form that asks for them is a form you have to read before using
+the app. Cancel exits before anything is allocated, because declining to choose a source is not a
+failure.
+
+It runs **before the model exists**, which is the whole reason it is cheap: no buffer, no socket
+and no window until a source is known, and `ScopeModel`, `Acquisition` and `run_ui()` are
+untouched by the feature. Every headless probe passes `--server`, so none of them reaches it.
+
+Two things the list had to get right. A session file is named `<host>_<port>.toml` and that name
+cannot say whether the source was multicast or which interface it joined on, so the file now
+records both — in the top block, before the first table, because a bare key written after a
+`[[table]]` header belongs to that table in TOML, which is how the `enabled` key was silently lost
+once already. And the list is **filtered to sessions that name a signal**: a development machine
+accumulates one file per probe run on a throwaway port — 140 of them here, every one with
+`signals = ["", "", "", ""]` — so an unfiltered chooser opens as a wall of ports nobody chose. A
+row's `×` forgets a source by deleting its session file, with no confirmation: the gesture is the
+confirmation, and what it destroys is one layout the app rewrites on the next clean exit.
+
 **Math channels.** A math channel is derived from the others by a graph of operations and is then
 an *ordinary channel*: the model synthesises a signal entry for it, so an enabled one is acquired,
 plotted, measured, exported and listed in the signal panel exactly like a streamed signal — and