feat: add multi line navigation actions

[viastolfi]

Closes #78.

Adds a numeric count prefix to normal-mode cursor motions, vim-style: 5j moves the cursor down 5 rows, 12G jumps to row 12, 5gg does the same from the top. Previously every motion only moved one row at a time, with no way to jump further without repeating a key or reaching for the mouse/search.

Changes

• New Count state machine (src/app/count.rs), modeled directly on the existing Chord: digits accumulate via push_digit and expire after the same LEADER_WINDOW idle timeout, so a stale prefix never leaks into an unrelated later keypress. Capped at 8 digits to stay well clear of u32 overflow.
• Digit keys (0-9) are intercepted in resolve_normal_key before the action match, so they never collide with existing single-key bindings.
• CursorDown/CursorUp treat the pending count as a repeat count; CursorTop/CursorBottom (gg/G) treat it as an absolute target line, clamped to the visible list bounds (no out-of-bounds cursor, no panic on an empty list).
• Status bar shows a " 12…" indicator while a count is pending, mirroring the existing chord indicator (" g…"); count is shown first since it's typed first (e.g. 5gg).
• Unit tests for Count covering accumulation, the 8-digit cap, and window expiry, following the same style as chord.rs's test suite.

Test plan

• cargo fmt --check
• cargo clippy --all-targets
• cargo test

All pass.

Notes

• HalfPageDown/HalfPageUp (Ctrl-d/Ctrl-u) intentionally don't consume a count yet — happy to wire that up too if useful, kept this PR scoped to the motions named in [feature request] Navigate by line number #78.
• No visible cap-reached feedback (typing a 9th digit is silently ignored); didn't feel worth a UI change for an edge case, but open to adding one.