* fix(chat): show fork controls for every provider
Real assistant messages require stable identity so provider-agnostic chat actions can address the selected turn. Preserve provider IDs where available and generate stable adapter-owned IDs otherwise.
* test(opencode): expect assistant message identities
* test(pi): cover assistant updates without message start
* fix(providers): preserve late assistant identities
* fix(opencode): track fallback compaction identities
