tui: Fix Shift+Tab kline sampling reverse cycle

[fengli-master]

? documents TAB, Shift+TAB as switching kline sampling both ways. Tab worked; Shift+Tab did nothing, on foot and any other terminal that sends CSI Z.

There is no existing issue for this.

Cause

crossterm 0.29 parses CSI Z (\e[Z, terminfo kcbt) as KeyCode::BackTab with KeyModifiers::SHIFT:

b'Z' => Some(Event::Key(KeyEvent::new_with_kind(
    KeyCode::BackTab,
    KeyModifiers::SHIFT,
    KeyEventKind::Press,
))),

The keymap bound BackTab with KeyModifiers::NONE and compared chords exactly. BackTab+NONE never equalled BackTab+SHIFT, so ActionId::BackTab was never dispatched and KlineType::prev() never ran.

KlineType::next() also clamps at Year rather than wrapping, so Tab cannot be used as a reverse path either.

The same miss happens on xterm, kitty, alacritty, and wezterm — not only foot. Kitty's CSI-u protocol reports Shift+Tab as Tab+SHIFT, which was also unmatched.

Fix

Fold the three encodings onto one chord in KeyChord::normalized(), the same way G and Shift+g are already folded:

• BackTab+SHIFT (crossterm 0.29 / CSI Z)
• Tab+SHIFT (kitty keyboard protocol)
• BackTab with no modifiers (older parsers)

Plain Tab is unchanged and still moves sampling forward.

Checks

• cargo fmt
• cargo clippy --bin longbridge -- -D warnings
• cargo test --bin longbridge tui::keymap (9 tests, including the new encoding cases)