Updates the document with the provided changeset. - Gas accounting (when and what is charged): - Base step charge (once per run of this step): 40 + 5 * changesetLength. - Expression/template resolution under "/changeset/**" (if present): - Expressions run in the deterministic QuickJS VM (same sandbox and bindings as the JavaScript Code step). - VM gas is converted to host gas as: hostGas = ceil(wasmFuel / 1700) (calibration factor currently 1700 fuel per host gas unit). - Any document(<pointer>) used inside expressions/templates charges per call: 8 + depth(pointer) + ceil(snapshotBytes/100). - Per change (applied in order after boundary validation): - Boundary check: 2. - Operation charge: - add or replace: 20 + ceil(valueBytes/100) (valueBytes is the canonical JSON size of val). - remove: 10. - Cascade routing (Document Update delivery): 10 * N, where N is the number of scopes that receive the resulting Document Update (origin scope and each participating ancestor up to root).