feat: fast navigation — gg/G (top/bottom) and Page Up/Page Down

[shouze]

Context

Part of the 🗂 TUI — Navigation & result management EPIC #66.

Currently the TUI only supports line-by-line scrolling (↑/↓). Power users expect Vim-style gg/G jumps and Page Up/Down for faster navigation through large result sets.

Acceptance criteria

• gg (two consecutive g keypresses) moves the cursor to the very first row
• G (capital G / Shift+G) moves the cursor to the last row
• Page Up / Page Down (and optionally Ctrl+U / Ctrl+D) scroll by a full visible page
• Scroll offset adjusts so the cursor is always visible after each jump
• Works correctly with folded repositories (skips folded rows)

Implementation notes

• gg requires detecting two consecutive g keys — a small stateful "pending key" buffer is sufficient (no need for a full key-sequence engine)
• G is a single keypress (\x47 or detected via key.name === 'G')
• Page height = terminal rows minus the fixed header/footer lines (already computed in the render loop)
• Pure helper clampCursor(rows, index) — keep in src/tui.ts or extract to src/render/rows.ts if reusable

Branch

feat/tui-fast-navigation → merge into feat/tui-navigation-result-management

Validation

bun test
bun run lint
bun run format:check
bun run knip