Function handle_create_block 

pub(super) fn handle_create_block(
    state: &mut State,
    tab_id: String,
    meta: Value,
) -> Vec<Event>

Phase E.3 — create a block inside a tab. Validates parent tab exists; otherwise emits Event::Error (non-fatal). On success: assigns a UUID, appends to the tab’s block_ids, inserts into state.blocks, emits Event::BlockCreated.

NOT idempotent on retry (UUID assignment per call); saga-side dedup is responsible for at-most-once delivery in E.5+.