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/settings.cpp

@@ -3,6 +3,7 @@
 
 #include "settings.hpp"
 
+#include <algorithm>
 #include <cstdlib>
 #include <cstring>
 #include <fstream>
@@ -64,6 +65,9 @@ Settings load_settings() {
       s.event_cache_max = std::max(0, std::atoi(value.c_str()));
     } else if (key == "relative_time") {
       s.relative_time = parse_bool(value, s.relative_time);
+    } else if (key == "memory_limit_mb") {
+      const long mb = std::strtol(value.c_str(), nullptr, 10);
+      if (mb >= 16) s.memory_limit_mb = static_cast<int>(std::min<long>(mb, 1000000000L));
     } else if (key == "theme") {
       if (value == "auto" || value == "light" || value == "dark") s.theme = value;
     } else if (key == "contextual_help") {
@@ -83,6 +87,7 @@ bool save_settings(const Settings& settings) {
   out << "event_cache_dir = \"" << settings.event_cache_dir << "\"\n";
   out << "event_cache_max = " << settings.event_cache_max << "\n";
   out << "relative_time = " << (settings.relative_time ? "true" : "false") << "\n";
+  out << "memory_limit_mb = " << settings.memory_limit_mb << "\n";
   out << "theme = \"" << settings.theme << "\"\n";
   // The two help keys are the *app's* memory rather than the dialog's: the contextual-help rows --
   // the Help menu's and the source chooser's own -- write the first, and the first-run tour writes the