docs: help, shortcuts, About and the two-tier status bar

Adds the four new features to README.md (About dialog, shortcut overview
with Ctrl/Alt key table and deviation note, 14-page help window, two-tier
status bar), adds the four new probes and six new invariants to CLAUDE.md,
and fixes the `10.0 kS/s` rate-format error in src/help/14-status.txt
(format_rate() uses %.3g, which prints `10 kS/s`).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -17,6 +17,11 @@ cmake --build build
                  --multicast 239.1.2.3 --interface 10.0.0.9  # multicast
 ```
 
+**System dependency: libadwaita ≥ 1.8** (`pkg_check_modules(ADWAITA ... libadwaita-1>=1.8)`).
+It is used for exactly one widget — the shortcut overview is an `AdwShortcutsDialog` — because
+GTK4's own `GtkShortcutsWindow` is deprecated as of 4.18 and `AdwShortcutsDialog` is its
+replacement.
+
 Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb`
 (size of the sample buffer, default 200), `--max-window` (largest time
 window the toolbar offers, default 600), `--holdoff` (default trigger hold-off),
@@ -1209,6 +1214,67 @@ on the live ring too. So exporting during a capture writes the partly filled win
 is on screen (`time` from `-pre` up to wherever the post side has reached), not the
 previous event.
 
+**The About dialog** (overflow menu) reports the running binary's version, the git commit it was
+built from, the build timestamp, the settings directory and this session's layout file. Version,
+commit and timestamp come from `src/build_info.cpp`, generated by CMake at configure time and
+regenerated on every build — so the commit and the timestamp always belong to the binary that is
+running, and `git` absent leaves the commit as `"unknown"` without breaking the build.
+
+**The shortcut overview** (toolbar **?** button, or Ctrl+?) lists all keyboard and mouse controls
+in two groups — application actions and plot controls — and is an `AdwShortcutsDialog` rather
+than GTK4's deprecated `GtkShortcutsWindow`. libadwaita ≥ 1.8 is therefore a hard dependency;
+`CLAUDE.md` records the one consequence worth knowing before editing (the stylesheet it installs
+reaches every widget). The full key table:
+
+| key | action |
+|---|---|
+| `a` | arm / disarm the trigger |
+| `Space` | pause / resume |
+| `t` | show / hide the trigger panel |
+| `c` | show / hide cursors |
+| `m` | show / hide measurements |
+| `g` | show / hide math channels |
+| `l` | show / hide the layout editor |
+| `s` | show / hide the side panel |
+| `f` | fit to data |
+| `e` | export visible range |
+| `+` / `-` | zoom in / out (both axes) |
+| `Ctrl+` / `Ctrl-` | zoom in / out (x axis only) |
+| `Alt+` / `Alt-` | zoom in / out (y axis only) |
+
+The wheel uses Shift = x / Ctrl = y, while the keyboard uses Ctrl = x / Alt = y. The two
+cannot agree: `+` *is* Shift+`=` on common layouts, so a bare zoom-in already arrives with
+`Mod::Shift` set, and using Shift for axis restriction would make x-only zoom unreachable from
+the keyboard. This is a **deliberate deviation from the wheel's convention**, documented in the
+shortcut overview itself.
+
+The six "Plot mouse" rows (wheel, drag, right-click) carry no accelerator key — GTK's shortcut
+syntax has no member for pointer gestures — and put the prose in the `subtitle` field instead,
+which `AdwShortcutsItem` renders as free text.
+
+**The 14-page help window** (overflow menu, F1, or the toolbar **?** button long-press) is a
+scrollable text view that opens on the page nearest to what the user was doing: the help pages
+are compiled into the binary as a GResource (`src/help/udpscope_help.gresource.xml`,
+`--c-name=udpscope_help`) for the same reason the icons are — udpscope is run straight out of the
+build tree with no install step, so a search path would have to be found at runtime and a missing
+file fails silently. The pages are Pango markup (`.txt` bodies), validated at load; a page that
+fails validation renders a one-line stand-in so the window always opens rather than crashing.
+`UDPSCOPE_PROBE_HELP_MS` verifies that `parsed == pages` and that the `closed ok` line carries
+no `Gtk-CRITICAL`.
+
+**The status bar is two tiers.** The bar itself shows one glance line: the activity badge, the
+display state (one word from a fixed set), the stored rate and the window. Everything else —
+packet and frame counts, gaps, buffer size and span, per-plot trace counts — is behind the ⓘ
+button at the bar's right-hand end, opening a popover. The two moved there because a bar that
+carried all of them was too wide to read while watching a signal, and because the bar sits
+directly above the plot: a field whose width tracked the stream would relayout the plot on every
+update. The rate field uses `%.3g` (`10 kS/s`, never `10.0 kS/s`), and the elapsed-time field
+steps to a coarser unit rather than growing digits.
+
+The exit dump still prints the full detail string (`mode with trigger on:` / `mode after
+go-live:`) because those strings come from `format_detail()` — the same path the popover
+reads — not from the label, which no longer carries those fields.
+
 ## Review findings not (yet) implemented
 
 These are known deviations, found by a spec/code review, that are deliberate for