Widens SpanPanelClientProtocol and adds SpanMqttClient.register_connection_callback
so consumers can subscribe to broker connection state transitions.
Fan-out behavior is added in the next commit.

…back

- Drop redundant bool annotation on _live to match surrounding style
- Document that registration is edge-only, no synthetic initial call

_on_connection_change now emits edge-only notifications to callbacks
registered via register_connection_callback. Duplicate states are
suppressed; exceptions in one subscriber do not break others.

_FakeBridge now inherits from AsyncMqttBridge and _FakeHomie from
HomieDeviceConsumer, bypassing __init__ to avoid I/O setup. This
makes assignments to client._bridge and client._homie pass strict
type checking without type: ignore or cast().

- Comment in _on_connection_change documents that re-subscribe runs on
  every connected=True event (including duplicates), while callback
  fan-out is edge-only.
- Add regression test locking in the resubscribe-on-duplicate-True
  behavior.
- Tighten the reconnect test's topic assertion to an exact match
  against WILDCARD_TOPIC_FMT.

… live

Replaces the silent stale-cache return with explicit failure. Three
distinct messages (client not connected / broker disconnected /
homie not ready) so logs self-diagnose. Breaking change: consumers
that called get_snapshot() while offline now receive an exception
they can catch to drive offline-state logic.

Document that field values are arbitrary and only object identity
is asserted, so future additions to the SpanPanelSnapshot dataclass
can be handled with zero/empty defaults without touching assertions.

Addresses final-review items:
- SpanPanelStaleDataError is now importable from span_panel_api directly,
  matching the pattern of every other exception in the hierarchy.
- close() resets the _live edge tracker so a subsequent connect() on
  the same instance correctly fires a True edge for the new session.

…or in README

- Update SpanPanelClientProtocol row in the protocols table to include
  register_connection_callback.
- Add a Connection State Monitoring section after Streaming Pattern
  showing how to subscribe to broker connection edges, with a note that
  get_snapshot() now raises SpanPanelStaleDataError when not live.
- Add SpanPanelStaleDataError to the Error Handling table and explain
  the semantic distinction from SpanPanelConnectionError. Update the
  usage example to catch it during normal operation.

…n liveness

A snapshot-debounce timer scheduled just before a bridge disconnect
could fire after the fact and dispatch a snapshot built from the
still-ready accumulator — the panel-die case where no \$state=disconnected
message is published. Push consumers (e.g. the span HA integration)
then saw a spurious live snapshot while the bridge was actually down.

Two complementary fixes:
- _on_connection_change(False) now cancels any pending _snapshot_timer.
- _dispatch_snapshot is now gated by the same liveness predicate as
  get_snapshot() (bridge connected AND homie ready); stale dispatches
  bail silently with a debug log.

Amends 2.6.0 in place (not yet released).

Pre-existing drop-by surfaced while reviewing the liveness-signal work.