Track state of connections, seats, and devices

[glamberson]

Implements the state tracking requested in #15: equivalents of
eis_client_state, ei_state, eis_seat_state, ei_seat_state,
eis_device_state, and ei_device_state from libei/libeis.

Changes

Three new enums in event.rs, re-exported from request.rs:

• ConnectionState — Connected / Disconnected
• SeatState — Active / Closed
• DeviceState — Paused / Resumed / Closed

Each connection, seat, and device wrapper tracks its own state via
Mutex-guarded fields, updated automatically as protocol events arrive
(client side) or as methods are called (server side).

Client side (EiEventConverter)

• Connection state set to Disconnected on EOF / disconnect.
• Seat state set to Closed when the seat's Destroyed event fires.
• Device state follows Resumed / Paused / Closed lifecycle.
• Tracks pressed_keys, pressed_buttons, and down_touch_ids on each
  device, clearing them on pause (matching libei semantics).

Server side (EisRequestConverter)

• Connection state derived from the existing AtomicBool.
• Seat and device state tracked through add_seat / remove,
  resumed / paused / remove calls.
• pressed_keys, pressed_buttons, and down_touch_ids tracked on
  each device, cleared on pause or remove.

Design notes

• Consolidated into three enums rather than six, since the client and
  server perspectives share the same state values.
• Input state (pressed keys/buttons/touches) included because libei ties
  this tracking to the pause/resume lifecycle -- they are inherently
  coupled.
• All new public methods documented; # Panics sections added where
  .lock().unwrap() is used, per clippy missing_panics_doc.

Closes #15