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>

todo.md

@@ -9,7 +9,7 @@
     - user is acquiring a signal with 1MSps and 100_000 samples per udp frame
     - last frame as number <N> and previous <N-2> in this case a 100_000 indexes must be skipped
     - time is simply: `((<N_frame> - <N_last>) * <n_samples_per_frame> - <sample_index>) / <sample rate>` (or something like that)
-- [x] (3) ~~named configurations~~ **done**: one directory per source holding `<name>.toml` per configuration plus `active`; a configuration carries a note and the startup settings (`buffer_mb`, `relative_time`, `capture_packets`); the chooser lists them and the menu's _Configurations…_ opens/saves/deletes them.
+- [x] (3) ~~named configurations~~ **done**: one directory per source holding `<name>.toml` per configuration plus `active`; a configuration carries a note and the startup settings (`buffer_samples`, `relative_time`, `capture_packets`); the chooser lists them and the menu's _Configurations…_ opens/saves/deletes them.
 - [x] (4) ~~optional udp frame storage for debugging of strange behaviours~~ **done**: `--capture-packets N`; see README and the design note in `docs/superpowers/specs/` for the ring file, the popover toggle and `tools/capture_to_pcap.py`.
 - [x] (5) ~~open exported binary waveforms to view/analyse~~ **done**: `--open FILE` and the chooser's **Waveforms** page (the files opened before, newest first, plus _Open…_); the `.npy`'s dtype names the columns, so a load restores the signals, the samples and the axis the export had, and every instrument works on it — see README and `docs/claude/invariants-data-path.md`.
 - [x] (6) ~~imporve right click menu on the plots~~ **done**: the plot's context menu uses the main menu's row style (`menu_button` + `.udpscope-menu`, asserted by `UDPSCOPE_PROBE_ROWMENU_MS`: uniform `label_x=26`, `h=18`, the shortcut flush right), and deleting a signal is a **Remove signal** submenu listing that plot's traces plus _All traces on this plot_.