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/main.cpp

@@ -24,7 +24,11 @@ int main(int argc, char** argv) {
     // The preferences fill in what the command line did not decide. `--relative-time` is an explicit
     // instruction for this run, so a preference can only turn the mode on, never off -- which is also
     // why this is read here, before the buffers are planned: the mode changes how they are filled.
-    if (udpscope::load_settings().relative_time) cfg.relative_time = true;
+    const udpscope::Settings prefs = udpscope::load_settings();
+    if (prefs.relative_time) cfg.relative_time = true;
+    // The memory limit is the same kind of statement, the other way round: the flag is the explicit
+    // instruction for this run and the preference only fills in when it was absent.
+    if (!cfg.memory_limit_specified) cfg.memory_limit_mb = static_cast<std::uint64_t>(prefs.memory_limit_mb);
   } catch (const std::exception& e) {
     std::fprintf(stderr, "udpscope: %s\n", e.what());
     return 2;