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>

tools/make-help-shots.sh

@@ -126,7 +126,7 @@ pick_other_window() {
 start_app() { # LOGFILE ENVS... ; extra args via APP_ARGS
   local log=$1; shift
   # shellcheck disable=SC2086
-  env "$@" "$app" --server 127.0.0.1 --port "$port" --buffer-mb 16 ${APP_ARGS:-} >"$log" 2>&1 &
+  env "$@" "$app" --server 127.0.0.1 --port "$port" --buffer-samples 512k ${APP_ARGS:-} >"$log" 2>&1 &
   app_pid=$!
 }