Bump PROTOCOL_VERSION from 7 to 8 to match server. Three changes:
1. Add dialogue_response field decoding (DialogueResponseEvent with
line_id, text, speaker_entity_id) from server #305/D-028.
2. Fix Interact encoding: server changed PlayerAction::Interact from
unit variant to struct variant with {target_entity_id, verb}.
Extract _encode_action() helper to handle this consistently.
3. Update all test assertions that checked version=7 or expected
Interact as a bare string.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>