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>

CLAUDE.md

@@ -564,6 +564,10 @@ Two lines of the self-check output are assertions in disguise:
   so the panes the tick rewrites are measured directly. The `[width-probe]` lines are the
   per-change detail behind the `axes.w changes` count, and a step over 100 px dumps the widget
   tree's wide subtrees — that is what the documented, intermittent ~320 px step wants read.
+- `[help] pages=14 parsed=14 shortest='Overview' 1381 B; shortcuts: app=12 plot=13; about version=… commit=… built=… settings=…; closed ok` (stderr, `UDPSCOPE_PROBE_HELP_MS`) — **`parsed` must equal `pages`** and no page may be empty: a missing resource or markup Pango rejects renders a one-line stand-in instead of failing, so the window *looks* fine and only this count notices. `settings=` must be a real path (the same one the About dialog shows). `closed ok` with **no `Gtk-CRITICAL`** is as much a crash test of three windows built, presented and closed in one run as it is a state check. Two of the three are `GtkWindow`s and close with `gtk_window_close()`; the shortcut overview is an `AdwDialog` and closes with `adw_dialog_close()`, cleared on `::closed`. `shortest=` is the page title — a page whose body was never compiled into the binary is absent rather than empty, and a title alone is the stand-in's shape.
+- `[keys] a: armed 0->1 | space: paused 0->1 | t: trigger panel: 0->1 | … | editable focused: 'm' refused=1` (stderr, `UDPSCOPE_PROBE_KEYS_MS`) — **every `X->Y` pair must differ** and **`refused` must be 1**. The refusal is the whole reason the controller is in the bubble phase: every bare-letter accel is also a character, so an accel that fires while a `GtkEditable` has focus makes the window entry unusable. A pair reading `0->0` is an action whose dispatch does not reach the handler a click reaches. Note the format is `<key>: <widget-description>: <before>-><after>` — the name passed to `before_after()` includes the widget's description, so `t: trigger panel: 0->1` is the correct form (not `t: trigger panel 0->1` without the colon before the values).
+- `[zoomkeys] ctrl+plus x=[..]->[..] y_held=1 | alt+plus y=[..]->[..] x_held=1` (stderr, `UDPSCOPE_PROBE_ZOOMKEYS_MS`) — **both `_held` must be 1 and both zooms must still happen.** Run it inside a pinned state, where a frame cannot re-frame the other axis for its own reasons: `UDPSCOPE_PROBE_PAUSE_MS=1500 UDPSCOPE_PROBE_ZOOMKEYS_MS=2200`. A `_held=0` is the modifier not reaching the plotter's controller; an unchanged range on the *zoomed* axis is the opposite failure, the key doing nothing. Ctrl restricts to x and Alt to y; Ctrl+Alt zooms neither (no special case needed — neither flag alone fires).
+- `status bar: glance=[● live · 1.09 MS/s · 2 s window] max_glance_chars=53 detail_fields=11 popover_rebuilds=0` (exit dump) — **`max_glance_chars` must stay small** (under about 60) and **`popover_rebuilds` must be 0**. The status row is directly above the plot, so this is the "layout must not depend on content" rule read from the bar's own side: the state field is one word from a fixed set, the rate field is `%.3g` (so `10 kS/s`, never `10.0 kS/s`), and the only field trigger mode adds is the elapsed time, which steps units rather than growing digits. `detail_fields` is the popover's label count, a literal, so adding a label without thinking about layout shows up here. The glance rate format matters: `%.3g` drops the trailing zero that a `%.4g` or a fixed-precision format would keep — a rate that reads differently in the dump and in the help page is a format disagreement, and `14-status.txt`'s example must match it.
 
 The probes are the regression net — when changing UI or plot-host behaviour, extend
 the probe/dump in `src/ui.cpp` rather than adding a separate harness.
@@ -1301,4 +1305,48 @@ far the largest file.
   out — a source whose streamer was merely down during one run would silently vanish from the
   only list it can be picked from, for ever. Keeping the old file loses nothing: a run that
   configured nothing had nothing worth persisting.
+- **The status bar is two tiers, and the dump reads the detail one.** `format_glance()` writes
+  the bar and `format_detail()` writes both the popover's prose and `App::status_detail`, which
+  is what `exit_probe_step3`/`exit_probe_step2` copy into `dbg_mode_live`/`dbg_mode_triggered`.
+  Those two used to read the *label*, and the label no longer carries `packets`, `gaps`,
+  `plot 1:` or the buffer figures — so a `gtk_label_get_text()` there would silently reduce
+  every documented `mode …:` assertion to four fields. The glance line's state field is **one
+  word from a fixed set** (`live`, `paused`, `armed`, `capturing`, `event held`, `disarmed`) for
+  the layout reason above; a seventh word, or a field whose width tracks the stream, breaks the
+  row the plot is laid out under.
+- **The ⓘ popover's widgets are built once and only their text is set.** It is visible when the
+  tick updates it, so a rebuild is a per-frame relayout of a widget on screen.
+  `update_status_popover()` only ever calls `gtk_label_set_text()`, and `popover_rebuilds` in
+  the dump is the assertion.
+- **The About dialog, the help window and the shortcut overview are opened by `show_about()`,
+  `show_help()` and `show_shortcuts()`, and each closes with the matching call.** The help window
+  and the About dialog are `GtkWindow`s: they close with `gtk_window_close()`, which emits
+  `close-request` so the handler that nulls the file-scope pointer runs. `gtk_window_destroy()`
+  does not emit that signal and must not be used — the math dialog fell into exactly this trap,
+  and the pointer then points at a dead widget until the next attempt to open the same window.
+  The shortcut overview is an `AdwDialog` — it is **not** a `GtkWindow` — closed with
+  `adw_dialog_close()` and cleared on `::closed` (there is no `::close-request` on `AdwDialog`).
+  One `GtkWidget*` (or `AdwDialog*`) per window, nulled in its close handler, is the shape that
+  prevents the double-open defect across all three.
+- **`build_info.cpp` is generated, never committed.** `cmake/write_build_info.cmake` runs at
+  configure time (so `add_executable` can name the file) and again on every build through
+  `udpscope_build_info` (so the commit and the timestamp belong to the binary that is running).
+  `git` absent leaves `kCommit` at `"unknown"` and the build succeeds. The About dialog and the
+  `[help]` probe both read `kVersion`, `kCommit` and `kTimestamp` from it; `settings=` in the
+  probe reads `config_dir()`, which is the path the session file is written to.
+- **The keyboard zoom's modifiers are not the wheel's.** `+`/`-` with **Ctrl** restricts to x
+  and with **Alt** to y, while the wheel keeps upstream's Shift = x / Ctrl = y convention. The
+  two cannot agree: Shift is unusable on the keyboard path because `key_from()` folds `'+'` and
+  `'='` onto `kKeyZoomIn` — `+` *is* Shift+`=` on common layouts, so a bare zoom-in already
+  arrives with `Mod::Shift` set. Ctrl+Alt together zooms neither axis, which needs no special
+  case. The guards are `deps/plotter`'s own `ctrl_plus_zooms_only_x` and `alt_plus_zooms_only_y`
+  tests, plus `UDPSCOPE_PROBE_ZOOMKEYS_MS`.
+- **A help page is one `.txt`, one `<file>` line in the GResource manifest, and one `kPages`
+  row.** The bodies are Pango markup in a second GResource (`--c-name=udpscope_help`, so its
+  generated symbols cannot collide with the icons'), validated with `pango_parse_markup()` at
+  load; a page that fails either lookup or parse renders a one-line stand-in and is excluded from
+  `parsed`. `UDPSCOPE_PROBE_HELP_MS`'s `parsed` must equal `pages`.
+- **`find_accel()` masks the modifiers before comparing.** Lock, Num-Lock and the pointer buttons
+  all arrive in `GdkModifierType`, so an unmasked compare matches no table row at all once
+  Caps Lock is on.
 - Every source file carries an SPDX MIT header; match it in new files.