Function handle_move_block 

pub(super) fn handle_move_block(
    state: &mut State,
    block_id: String,
    src_tab_id: String,
    dst_tab_id: String,
    dst_index: u32,
) -> Vec<Event>

Phase E.5.5 — move a block from src_tab_id to dst_tab_id at dst_index (clamped). Updates block.tab_id. Cross-tab moves AND intra-tab repositioning both go through this command (the caller specifies the destination index regardless).

Errors when source / dest tab missing, block missing, or block.tab_id != src_tab_id.