Assert density was collapsing exactly where the working tree and the wire are touched: update.zig held 2 asserts across 2127 lines, serve.zig 3, transport.zig 2 -- against the hundreds in the artifact and sync cores. This pass adds 17 asserts stating invariants that already hold: the checkout-state transitions in update (managed-set, record and rollback image agreement around each applyCheckin stage), the request/response postconditions in serve (media, arity and body-ceiling), and the frame, redirect and body-cap postconditions in transport. Asserts only, no behaviour change; where a result had to be named to assert on it the error mapping is preserved exactly. All survive the deterministic simulation sweeps, which is the point of landing them before the checkout crash-recovery sweep that will hammer update. check-in: 9654d06838 user: dmestas tags: trunk