Function modify_store 

pub fn modify_store(
    store: &DataStore<DigDataStoreMetadata>,
    owner: StoreOwner,
    new_root: Bytes32,
) -> DigStoreResult<MerkleCoinSpend>

Spends the store’s tip coin to recreate it anchoring new_root — a new generation (SPEC §3.2).

store is the current confirmed tip (from crate::get_store_singleton_tip); the spend consumes it and recreates the singleton with new_root, PRESERVING every other anchored metadata field (label, description, size bucket, program hash) and the store identity (store_id, owner, delegation set). Returns the UNSIGNED spend.

Note: attaching a reserve fee to a modify spend is a dig-merkle future unit (its fee module is a documented stub); this builder recreates the coin at its current amount.

Errors

Returns a DigStoreResult error if the spend cannot be constructed.