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>

docs/claude/invariants-session-and-export.md

@@ -145,7 +145,7 @@ expression of the scheme, and the name is `sanitize()`d exactly as the host is,
 with a slash cannot escape the directory.
 
 A configuration carries the whole session *and* the settings that can only be decided before the
-first socket: `buffer_mb`, `relative_time`, `capture_packets`. The startup chooser applies them
+first socket: `buffer_samples`, `relative_time`, `capture_packets`. The startup chooser applies them
 (`apply_source()`), which works because `choose_source()` runs before `ScopeModel` is constructed
 — the one moment a buffer size can be applied at all. Switching configurations inside a running
 app applies what can change live and says in the status line that the rest takes effect on the
@@ -156,9 +156,9 @@ A configuration's *startup* settings reach the model from two directions, and bo
 named with `--server`, which never reaches the chooser at all. Without the second, a saved host's
 buffer size, clock and capture ring were silently ignored — the layout still applied, because the
 app loads that separately once a CONFIG names the signals — so an edit to any of the three read as
-one that never took effect. **A flag the user typed wins over the file** (`--buffer-mb`,
+one that never took effect. **A flag the user typed wins over the file** (`--buffer-samples`,
 `--relative-time`, `--capture-packets`): the file fills in what the command line did not say, and
-`Config::buffer_mb_specified` is what tells "not asked" from "asked for the default".
+`Config::buffer_samples_specified` is what tells "not asked" from "asked for the default".
 
 The `active` file is written on every save and by the chooser's pick, so "which configuration was
 I on" survives a restart. A session written before configurations existed is **imported once** as