Budget the sample buffer in samples per signal, under a hard memory limit

--buffer-samples (k/M/G) replaces --buffer-mb: the request is rounded down to a
power of two for every signal and the shared time ring. A separate memory limit
(--memory-limit-mb, a Preferences row, default 8 GiB) halves the plan until the
real ring bytes fit and says so on stderr and in the status line. The per-source
key buffer_mb becomes buffer_samples; an old buffer_mb is ignored.

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

src/layout_store.hpp

@@ -139,9 +139,12 @@ struct LayoutState {
   /// The startup settings the configuration carries, applied when the chooser opens it -- which is
   /// why they live here and not only on the command line: two sources on one machine want different
   /// buffers and different clocks, and retyping the flags to look at the second one is how a
-  /// setting gets forgotten. A zero `buffer_mb` and an empty `capture_packets` mean "not stored":
-  /// the command line's value stands, so an old file cannot silently shrink a buffer.
-  double buffer_mb = 0.0;
+  /// setting gets forgotten. A zero `buffer_samples` and an empty `capture_packets` mean "not stored":
+  /// the command line's value stands, so an old file cannot silently shrink a buffer. The budget is
+  /// samples per acquired signal (the memory limit is the application's, not a source's). A file from
+  /// before that unit carries `buffer_mb`, which is not read: it reads as "not stored" and the
+  /// default stands, rather than a megabyte count being taken for a sample count.
+  std::uint64_t buffer_samples = 0;
   bool relative_time = false;
   std::uint32_t capture_packets = 0;
 };
@@ -162,7 +165,7 @@ struct KnownSource {
   std::string config;
   std::string comment;         ///< the configuration's note, shown beside its name
   /// The startup settings this configuration carries, so the chooser can apply them.
-  double buffer_mb = 0.0;
+  std::uint64_t buffer_samples = 0;
   bool relative_time = false;
   std::uint32_t capture_packets = 0;
 };