list the sources with a stored session

known_sources() reads the session files the layout store already owns, keeping
only those that name a signal: a development machine accumulates one file per
probe run on a throwaway port, and all 140 of them here hold empty slots.

src/layout_store.hpp

@@ -53,6 +53,28 @@ struct LayoutState {
   std::vector<Measure> measures;
 };
 
+/// A data source udpscope has a stored session for: one row of the startup chooser.
+struct KnownSource {
+  std::string host;            ///< the CONFIG server, even for a multicast session
+  std::uint16_t port = 0;
+  std::string multicast;       ///< empty = unicast
+  std::string interface_addr;  ///< empty = the default route
+  int rows = 0;                ///< plots the session had
+  int signals = 0;             ///< named slots across those plots
+  std::int64_t mtime = 0;      ///< orders the list; never shown
+};
+
+/// Every session file that names at least one signal, newest first.
+///
+/// The filter is what makes the list usable: a development machine accumulates one session file
+/// per probe run on a throwaway port -- 140 of them when this was written -- and every one holds
+/// `signals = ["", "", "", ""]`. A listed source is one somebody actually configured.
+std::vector<KnownSource> known_sources();
+
+/// Deletes one source's session file; the chooser's `x`. False means the file was already gone or
+/// could not be removed, which costs the caller nothing -- there is no state to roll back.
+bool forget_source(const KnownSource& src);
+
 /// One `MathNode` as `kind|source|in_a|in_b|gain|offset|fc|q|lo|hi|x|y`, and back. The kind is the
 /// op's *index* into `math_op_at()`, which is why the enum's order is a stored format and not just
 /// an implementation detail: reordering it renames every saved graph's operations.