[P0] Nested map updates do not write back outer map (lost update)

[ngmachado]

Problem

Assignments like allowances[owner][spender] = amount lower to map_store on the inner map value, but the updated inner map is not written back into the outer map.

Impact

approve / transferFrom postconditions over allowances fail; runtime storage semantics are incorrect for nested maps.

Acceptance Criteria

Nested map assignment lowers to:

• load outer map value,
• update inner map,
• store updated inner map back into outer map.

Add regression for map[address, map[address, u256]] write/read roundtrip.