fix(bitcoind_rpc): emit invalidated heights when start_height is above agreement...

fix(bitcoind_rpc): emit invalidated heights when start_height is above agreement point

When a reorg drops the agreement point below `start_height`, the emitter
would skip directly to `start_height`, producing a checkpoint that could
not connect with the caller's local chain (`CannotConnectError`).

Fix: override `start_height` to the agreement height when a reorg is
detected (agreement point below both `start_height` and `last_cp`), so
the emitter revisits the invalidated block heights.