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/window.page

@@ -56,11 +56,10 @@
 
   <section id="example">
     <title>Example</title>
-    <p>1 MS/s with 200 MB of buffer. A 2 s window stores every sample
-    (<code>time x1</code>). Widen it to 600 s and the stride becomes <code>x16</code>: 625 S/s
-    stored, anti-aliased at 312.5 Hz. A 2 kHz sine seen through that window reads
-    <code>y=[-0.059, 0.079]</code> — attenuated to about 6%, not folded down to a
-    full-amplitude 125 Hz one.</p>
+    <p>1 MS/s with the default buffer of 8 388 608 samples per signal (8.39 s). A 2 s window
+    stores every sample (<code>time x1</code>). Widen it to 600 s and the stride becomes
+    <code>x128</code>: 7.8 kS/s stored, anti-aliased at 3.9 kHz. A 5 kHz sine seen through that
+    window is attenuated by the filter, not folded down to a full-amplitude low-frequency one.</p>
   </section>
 
 </page>