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/preferences.ui

@@ -13,7 +13,7 @@
   show-apply-button is off here: the write is a few bytes to one file, so a keystroke is not worth
   batching). The two settings that cannot take effect immediately — the cache directory and relative
   time, both of which are planned before any source is known — say so in their subtitles instead of
-  being held back by a button.
+  being held back by a button. The memory limit does take effect at once: a running buffer re-plans.
 
   Nothing here carries a value: `ui_prefs.cpp` fills the rows from `Settings` when the dialog opens,
   and writes it back on every change. This file is the shape, that is the state.
@@ -77,6 +77,27 @@
             </child>
           </object>
         </child>
+        <child>
+          <object class="AdwPreferencesGroup">
+            <property name="title" translatable="yes">Sample buffer</property>
+            <child>
+              <object class="AdwSpinRow" id="memory_row">
+                <property name="title" translatable="yes">_Memory limit (MB)</property>
+                <property name="use-underline">True</property>
+                <property name="subtitle" translatable="yes">Hard ceiling on the memory of the sample rings, all signals together. A source asks for a number of samples per signal; if that does not fit under this limit it is halved until it does, and the status line says so. Applies at once: a running buffer is re-planned, which clears its history.</property>
+                <property name="adjustment">
+                  <object class="GtkAdjustment">
+                    <property name="lower">16</property>
+                    <property name="upper">1048576</property>
+                    <property name="step-increment">256</property>
+                    <property name="page-increment">1024</property>
+                    <property name="value">8192</property>
+                  </object>
+                </property>
+              </object>
+            </child>
+          </object>
+        </child>
       </object>
     </child>
     <child>