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-measurements.md

@@ -121,7 +121,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
 - **A pass costs the window's buckets times the pane's *measured channels* and its *distinct
   levels*, and the window's own size multiplies all of it.** The cadence above bounds *how
   often*; this is *how much*, and it is the term a big time scale buys. Measured on the mock at
-  the user's scale -- a **15 s** window, `--buffer-mb 4000` (so `time_stride` stays 1 and the
+  the user's scale -- a **15 s** window, `--buffer-samples 100M` (so `time_stride` stays 1 and the
   window is 11.8 M samples, 184 k level-0 buckets), four signals, one row -- with
   `[levels-probe]` (see [verification.md](verification.md)), `measure cost:` reading
   `recomputes`/`each`/`covered`/`buckets`/`raw`: