docs: settle source-plugin implementation details in the spec

docs/superpowers/specs/2026-10-31-source-plugins-design.md

@@ -42,19 +42,24 @@ later plugins need.
   (a TOML table), the `--source-opt` form, and what a config widget reads and writes.
 - **`Frame`**: source-neutral, mirrors `udps_frame_t`: `counter`, `lost`, `late`,
   `recv_time`, `publish_mode`, `num_samples`, `signals[]` (the existing `SignalInfo`) and
-  `values[]` (doubles). The hole, backfill and time-base logic needs `counter/lost/late`; a
+  `values[]` (doubles). `signals` and `values` are pointers into the source's own buffers,
+  valid only for the duration of `on_data`; `Acquisition` copies what it keeps. The hole, backfill and time-base logic needs `counter/lost/late`; a
   source with no counter supplies a running counter and `lost = 0`. `Frame::value(sig, k,
   col)` replaces `udps_frame_value`.
 - **`DataSource`**: the per-connection object.
   - `open()`, `poll(timeout_ms)`, `close()`.
   - `stats()` returns the counters now in `StreamStats`; `connected()` reports link state.
-  - `post(Command)`: thread-safe queue, drained inside `poll()`.
+  - `post(Command)`: thread-safe queue, drained inside `poll()`. `Command` is
+    `std::function<void()>`.
   - `set_datagram_tap(fn)`: optional; lets UDP-based sources feed the raw-packet capture.
   - Output goes to a `SourceSink` (`on_config`, `on_data`, `on_event`) called inside
     `poll()`, as `udps_client` does today.
 - **`SourceKind`**: factory and UI description of one plugin type: `id`, `display_name`,
-  `identity(cfg)`, `make(cfg)`, `make_config_form(cfg)` (a `GtkWidget*` plus a `read()`
+  `identity(cfg)`, `make(cfg)`, `make_config_form()` and `make_panels(source)` (each
-  closure) and `make_panels(source)` (each `{id, title, GtkWidget*}`).
+  `{id, title, GtkWidget*}`). `make_config_form()` takes no argument: the chooser calls
+  `form.write(cfg)` to fill the form. A `ConfigForm` is a list of `AdwPreferencesRow` widgets
+  plus `write` / `read` / `valid` / `on_change` closures, so the chooser can place the rows in
+  its own group and gate its Connect button on `valid()`.
 - **`SourceRegistry`**: filled by an explicit `register_builtin_sources()` call. No
   static-init self-registration (linkers can drop it). A future `dlopen` loader would only
   call `register_kind()`.
@@ -62,7 +67,8 @@ later plugins need.
 ### 2. Acquisition and the UDPS plugin
 
 - `Acquisition` takes a `std::unique_ptr<DataSource>` instead of building a `udps_client_t`
-  and implements `SourceSink`; the four trampolines go away.
+  and implements `SourceSink`; it creates the source lazily in `start()`, from the kind and
+  `SourceConfig` it was given; the four trampolines go away.
 - `on_config` / `on_data` change only in the type they read. Time base, holes, backfill,
   decimation, trigger and math code are untouched.
 - `run()` calls `source_->poll(50)` and copies `source_->stats()` into
@@ -81,7 +87,8 @@ later plugins need.
   interface) moves into the UDPS plugin. The dialog keeps the known-sources list, buffer
   size, relative time and capture ring size.
 - **Capability panels**: after the source is created the app calls `make_panels()` and adds
-  each panel as a page of a "Source" stack in the side area. A panel is insensitive when
+  each panel as a page of the existing `side_stack` (the side area's page stack, owned by
+  the tab switcher). A panel is insensitive when
   disconnected (disabled, not hidden; the tab switcher owns visibility). A source with no
   panels adds no tab; UDPS has none. Panels call `source->post(Command)`, never the source
   directly, so cross-thread safety lives in one place. A helper kit for common panels (DIO
@@ -90,8 +97,10 @@ later plugins need.
 - **Config / CLI**: `Config` gains `source_kind` (default `udps`) and a `SourceConfig`.
   `--server/--port/--multicast/--interface` populate the UDPS entries. `--source NAME` and
   repeatable `--source-opt k=v` work for any kind. `--help` lists the registered kinds.
-- **Sessions**: file `<kind>_<identity>.toml` with the source's config in a `[source]`
+- **Sessions**: the source's config is stored as top-level TOML keys, `source_kind` and
-  table. Legacy `host_port.toml` is read as `udps`; nothing is migrated destructively.
+  `source_opt_<key>`, written only for non-UDPS kinds (a UDPS session file is unchanged, so
+  legacy `host_port.toml` files read as `udps` with nothing migrated). It is not a `[source]`
+  table. A non-UDPS session file is named by a pseudo-host `<kind>-<identity>` with port 1.
 - **Mocks**: `tools/mock_udpstreamer.py` remains the UDPS mock. Each later plugin ships its
   own mock under `tools/` so the headless self-check keeps working.
 
@@ -99,7 +108,9 @@ later plugins need.
 
 - This is a pure refactor for UDPS: the existing headless self-check must read exactly as on
   `main` -- exactly three build warnings, no `Gtk-CRITICAL` (see `docs/claude/verification.md`).
-- New probe `UDPSCOPE_PROBE_SOURCES`, extending the probe/dump in `src/ui.cpp` rather than
+- New probe `UDPSCOPE_PROBE_SOURCE_KINDS` (`UDPSCOPE_PROBE_SOURCES` is already the chooser
+  discovery probe). Its value is a delay in ms, and its presence also registers the stub
+  `probe` kind. It extends the probe/dump in `src/ui.cpp` rather than
   adding a harness. It checks that the registry lists its kinds, that a config form
   round-trips its `SourceConfig`, and -- with a stub test-only kind carrying one panel --
   that the panel's tab is built, insensitive while disconnected, and that a posted command is