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>

CLAUDE.md

@@ -6,7 +6,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 udpscope is a **GTK4 oscilloscope-style viewer for a UDPStreamer source**. It receives one
 data source -- a UDPStreamer stream through `deps/udpsclient` by default, or another registered
-source kind (see `docs/claude/architecture.md`) -- buffers it in a large ring (default 200 MB), and draws it
+source kind (see `docs/claude/architecture.md`) -- buffers it in a large ring (default 10 M samples per signal, under an 8 GiB memory limit), and draws it
 with `deps/plotter` — a figure of up to eight stacked rows sharing one x window. On top of
 the live view it provides the things a bench scope provides: a trigger with a pre/post event
 window, a pause that freezes the visible window, cursors, statistical measurements,
@@ -19,6 +19,11 @@ Two documents bound the work and are **not** interchangeable with this one:
 - **`README.md`** documents what was actually *built*, including deliberate deviations from
   the spec and known limitations. **Read it before concluding something is a bug.**
 
+**Deliberate deviation from `SPECS.md` line 1:** the sample buffer is budgeted in *samples per signal*
+(`--buffer-samples`, rounded down to a power of two), not in megabytes, under a separate hard
+`--memory-limit-mb` (also a Preferences row). There is no `--buffer-mb`; see README and `SPECS.md`'s
+"Deviations from the spec".
+
 Three things shape almost every change here, and the linked files expand on each:
 
 - **There are exactly two threads** — a producer that owns the data source and the GTK main