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_probe.cpp

@@ -1182,7 +1182,7 @@ gboolean math_probe(gpointer user) {
 // The one state `math_probe` structurally cannot reach: an edit of a graph whose ring has already
 // *wrapped*. Every edit that probe makes follows a structural change, which replans the buffer and
 // restarts every ring at absolute 0 -- and at `base == 0` the recompute's two renumbering bugs are
-// both identities. Run it with a small `--buffer-mb` and give it time to saturate; `base=0` in its
+// both identities. Run it with a small `--buffer-samples` and give it time to saturate; `base=0` in its
 // second line means the run was too short or the buffer too large to have read anything.
 int g_mathwrap_index = -1;
 double g_mathwrap_y_before = 0.0;
@@ -1231,7 +1231,7 @@ gboolean mathwrap_probe_wait(gpointer user) {
       g_mathwrap_y_before = a->model->live().channel(idx).y_bounds().max;
     }
   }
-  // Polled rather than timed: the wrap is a function of the rate and of `--buffer-mb`, and a fixed
+  // Polled rather than timed: the wrap is a function of the rate and of `--buffer-samples`, and a fixed
   // delay that fell short would print a passing line having tested nothing.
   if (base == 0 && ++g_mathwrap_polls < 200) return G_SOURCE_CONTINUE;
   std::fprintf(stderr, "[mathwrap-probe] wrapped: base=%llu slots=%llu (want base > 0)\n",
@@ -5384,9 +5384,10 @@ gboolean configs_probe(gpointer user) {
   const double noise_back = st.measures.empty() ? -1.0 : st.measures.front().options.noise_pct;
   const double hf_back = st.measures.empty() ? -1.0 : st.measures.front().options.hf_fc;
   std::fprintf(stderr,
-               "[configs] round trip read=%d comment='%s' buffer_mb=%.6g relative=%d "
+               "[configs] round trip read=%d comment='%s' buffer_samples=%llu relative=%d "
                "capture=%u plots=%zu signals=%d measures=%zu math=%zu noise=%.6g hf=%.6g\n",
-               read ? 1 : 0, st.comment.c_str(), st.buffer_mb, st.relative_time ? 1 : 0,
+               read ? 1 : 0, st.comment.c_str(),
+               static_cast<unsigned long long>(st.buffer_samples), st.relative_time ? 1 : 0,
                st.capture_packets, st.rows.size(), signals, st.measures.size(), st.math.size(),
                noise_back, hf_back);