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/overview.page

@@ -18,7 +18,7 @@
     <p>udpscope plots a live UDPStreamer source. One window holds up to eight rows of plots,
     each with up to four traces, and every row shares one time axis, so the whole figure
     scrolls together. Acquisition runs on its own thread and writes a ring buffer sized from
-    <cmd>--buffer-mb</cmd>; the drawing thread reads whatever that ring holds when a frame
+    <cmd>--buffer-samples</cmd>; the drawing thread reads whatever that ring holds when a frame
     starts, which is why a trace never makes the window stutter.</p>
 
     <p>The stream is the clock. Every interval you read on screen is measured between