mint

Introduce root supply at root: credit holder with amount units under mintId. Control-plane only (design §9); the one non-conserving op and the only mutator with no feasibility gate, so it never returns null. mintId MUST be unique per mint act so distinct acts union rather than max-collide (design fix 4).

root must be the ledger's one root: the supply is creditable only at the group it names (#1751), so minting at any other group strands the units where holdings never reads them — and validate reports the resulting two-rooted state as LedgerConflict.MultipleRoots.