feat(input): negotiate keyboard enhancement protocol when supported

[laurigates]

What

Negotiate the keyboard enhancement ("kitty") protocol at startup when the
terminal advertises support, and pop it on teardown.

jnv enables raw mode but never requests the protocol, so supporting terminals
fall back to legacy encoding. In legacy mode Ctrl+J is indistinguishable from
Enter (both 0x0A) and Ctrl+I from Tab, so the JSON viewer's default
down = ["Down", "Ctrl+J", "ScrollDown"] binding can never fire via Ctrl+J
(it lands on Enter/toggle instead), and modified/bracket keys are reported
inconsistently across terminals.

How

• main.rs: when supports_keyboard_enhancement() reports support, push
  KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES; TerminalCleanupGuard
  pops it on drop. Terminals without support keep the exact prior behavior.
• event_dispatcher.rs: keybinds are matched by exact Event equality, so
  enabling the protocol requires canonicalizing incoming events. Added a pure
  normalize_event helper at the single event-dispatch chokepoint (covers every
  downstream matcher) that drops non-Press events (releases/repeats become
  possible under the protocol) and strips KeyEventState lock bits (Caps/Num
  Lock) so a binding matches regardless of lock state. The existing scroll
  position-zeroing moved into the same helper. Unit-tested.

Deliberately requests only DISAMBIGUATE_ESCAPE_CODES — not
REPORT_EVENT_TYPES/REPORT_ALL_KEYS — to keep the event stream close to legacy
(no release-event noise) while fixing the ambiguous-control-key class.

Verification

cargo fmt --all -- --check, cargo clippy, and cargo test all pass.
Manually verified on kitty (macOS): Ctrl+J/Ctrl+K now scroll the JSON viewer
as bound; bracket entry on a Finnish layout is unaffected.

Notes / open questions

This is offered as "here's what made jnv work better for me on kitty" — happy to
adjust or drop it if it doesn't fit the roadmap.

• I have not tested behavior under a terminal multiplexer (tmux/screen),
  where supports_keyboard_enhancement() and protocol passthrough are
  historically finicky. If you have tmux users, that path may warrant a look.
• Enabling the protocol is a behavior change on all enhancement-capable
  terminals (Ctrl+J≠Enter, Ctrl+I≠Tab). That's the intended fix, but
  flagging it explicitly.
• The normalize_event approach keeps the existing exact-Event matching; if
  you'd prefer a different matching strategy, glad to rework.