v3.4.0

external-attrs contract — the end of multi-shade attribute ping-pong

Completes the ÷2 double-write fix from v3.3.x and removes the display freeze
that happened while several shades were moving at once.

• Extra attributes injected by a third-party component (cover-extender, for
  instance) now travel through a shared contract
  (hass.data['cover_external_attrs']) instead of a cross state rewrite.
  No more attribute round-trips multiplying state_changed events.
• Dynamic reader handshake: the third-party component deposits its extras,
  the cover entity soft-merges them and rewrites itself once, on a dispatcher
  signal.
• Works with the third-party component absent — the read is soft and has no
  impact.
• Still one entity per shade.

Tests: the pytest suite passes, including the end-to-end run against a real
Home Assistant.