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/ui_prefs.hpp

@@ -62,6 +62,7 @@ struct PrefsForm {
   AdwEntryRow* dir_row = nullptr;
   AdwSpinRow* keep_row = nullptr;
   AdwSwitchRow* relative_row = nullptr;
+  AdwSpinRow* memory_row = nullptr;
   AdwComboRow* theme_row = nullptr;
   /// The folder chooser beside the directory row. A text field alone asks a user to know a path and
   /// type it correctly; the chooser is how they find one, and what it returns goes into that row -- so