docs: the sample budget in samples per signal, the memory limit and the planner probe lines

Converts every --buffer-mb / buffer_mb mention to --buffer-samples / buffer_samples,
records the unit change as a deviation (README, SPECS, CLAUDE.md), documents the
buffer: and planner: dump lines and the new Preferences row, and updates the help
pages and the help-shot script.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

help/C/org.udpscope.app/sessions.page

@@ -41,7 +41,7 @@
     they open.</p>
 
     <p>A configuration also carries the settings that can only be decided when the buffers are
-    allocated: the <gui>buffer size</gui> (<cmd>--buffer-mb</cmd>), relative time, and the
+    allocated: the <gui>samples per signal</gui> (<cmd>--buffer-samples</cmd>), relative time, and the
     packet capture's ring size. Opening a configuration from the chooser applies all three;
     switching inside a running app applies the rest and says in the status line that these take
     effect on the next start.</p>
@@ -57,8 +57,8 @@
     <list>
       <item><p>What a configuration persists: the time window, the rows and each row's slot
       signals, the calibration, the trace colours, which signals were enabled, the math
-      channels with their graphs, the measurements pane — and the startup settings: buffer
-      size, relative time and the packet capture's ring size.</p></item>
+      channels with their graphs, the measurements pane — and the startup settings: samples
+      per signal, relative time and the packet capture's ring size.</p></item>
       <item><p>Signal identity is the <em>name</em> throughout, because a new CONFIG renumbers
       the channels.</p></item>
       <item><p>A file written by a run against a source whose streamer was down would name no