The ProducerEvent split is the right shape for #857's step 3: every producer exit path I traced (size limit, model error, cancelled-then-answer, fallback, empty answer, save failure, success) now yields an outcome and the spawn loop emits exactly one terminal, with Closed-without-terminal correctly surfaced as an error given how live::emit freezes the snapshot. Two changes before merge:

1. Drop commit b8fec3d2 (vector index drop/build lock in crates/utopia-store/src/vector_index.rs and its test) into its own PR. It is unrelated to #857 and undocumented here; it looks reasonable on its own and deserves its own description.
2. The terminal table in chat_terminal_tests.rs lost its pending list and the injection-point section without gaining rows. The targeted tests do exist on dev (chat_persistence_tests.rs, chat_fallback_tests.rs, chat_registry_tests.rs), so either add the rows or name those tests in the comment so the table stays the place reviewers look.

Optional: make Progress a type that cannot hold a terminal event (today a done frame sent as progress is only caught at runtime), and put the saved message id on done so a client can verify the "earned" claim. The parser swap and its CR/CRLF tests look good.

Both review points applied: the vector-index commit is #908 on its own, and the terminal table names the targeted test files; 48 chat tests, clippy, and the web suite pass locally; CI green. Approving to clear the earlier request-changes.