feat(ui): demo-mode keystroke badges (#22, ADR-0047 D2/D4/D5)

In --demo mode, an otherwise-invisible key (Tab, Enter, arrows,
Ctrl-O, …) raises a transient [LABEL] badge — a floating
black-on-yellow box inset at the output panel's bottom-right. Set in
App::update before the modal gate (so it shows over the load picker,
the #24 cast); pure demo_badge_label maps the key set. The runtime
expires it on a ~1.5s timer via a new nearest_deadline helper that
extends the existing time-boxed-recv arm condition without disturbing
the ADR-0027 indicator debounce. New App.last_output_area lets the
top-level draw anchor the overlay; overlay colours centralised in
theme.rs.

Tier 1 (label fn, badge set/seq, over-modal), Tier 2 (dark/light
snapshots, black-on-yellow style, too-small clamp), runtime unit
(nearest_deadline). Phase B of ADR-0047; captions land in C.

src/theme.rs

@@ -20,6 +20,16 @@ use ratatui::style::Color;
 
 use crate::dsl::grammar::HighlightClass;
 
+/// Foreground of the demonstration-mode overlays (ADR-0047 D4).
+///
+/// Deliberately a fixed, theme-independent high-contrast pair — black
+/// on yellow — so the badge / caption boxes are hard to overlook in a
+/// screencast on any background.
+pub const DEMO_OVERLAY_FG: Color = Color::Black;
+/// Background of the demonstration-mode overlays (ADR-0047 D4); see
+/// [`DEMO_OVERLAY_FG`].
+pub const DEMO_OVERLAY_BG: Color = Color::Rgb(0xFF, 0xD7, 0x00);
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Background {
     Light,