Function rope_replace_block_content 

pub fn rope_replace_block_content(
    store: &Store,
    block_id: EntityId,
    new_text: &str,
)

Replace the entire content of a registered block in the rope with new_text. Preserves the block’s byte_start and its trailing boundary \n (if any); subsequent entries shift by the net length delta.

Used by use cases that compute a block’s final content as a string and want to push that content to the rope in one shot — e.g. the block-splitting branches of insert_html_at_position_uc and insert_markdown_at_position_uc, where each affected block (head, tail, mid-replacement) gets a single new value.

No-op if block_id is not in the index.