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-data-path.md

@@ -20,7 +20,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   comparison on purpose: it is not a size, but it is what the slots *mean*, and keeping samples
   under a changed `base_dt` would re-date the whole ring.
   **Construction is the cost.** A `RingBuffer<T>` holds a `std::vector<std::atomic<T>>`, which is
-  value-initialised, so making a store touches every byte of `--buffer-mb` — about a second per
+  value-initialised, so making a store touches every byte of the planned rings — about a second per
   gigabyte, mostly first-touch page faults — while `reset()` is one relaxed store per ring plus an
   `invalidate()` and an epoch back to zero. It must leave nothing of the old history behind, which
   is why it clears the time ring, the absolute-time ring, every value ring, and both the published
@@ -65,8 +65,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   trade and is why the status line's `rate` field is worth reading.
 - **A file is ingested too, so the loader decimates by the same factor.** `load_waveform_into_model()`
   sets the target span to the recording's own length and enables every column, and the plan that
-  comes back may well decimate: 20 s of 10 channels at 500 kS/s is 480 MB of rings, and the budget is
-  `--buffer-mb`. The loader therefore reads `partition().time_stride` **after** the enables and pushes
+  comes back may well decimate: 20 s of 10 channels at 500 kS/s is 10 M samples a channel, and the budget is
+  `--buffer-samples` (rounded down to a power of two, 8.39 M by default). The loader therefore reads `partition().time_stride` **after** the enables and pushes
   one slot per that many rows, carrying the group's mean at the group's mean stamp — the packet path's
   filter, written a second time because a file does not arrive in packets. Pushing every row instead
   overruns the ring and leaves the file as **its own tail**, with nothing to say so: a 20 s event