Snapshot/commit semantics for nested EVM call frames.

When a child frame halts normally (:stopped), its mutations to the parent's revertible fields are promoted; on any other halt they are discarded and the parent observes the pre-call state.

Revertible fields (the db plus all of substate) participate in the merge:

• db — the unified world database (accounts + storage)
• substate.logs — emitted log entries (parent's first, then child's)
• substate.accessed_addresses — EIP-2929 access list
• substate.accessed_storage_keys — EIP-2929 access list
• substate.created_addresses — EIP-6780 set of contracts created in this tx
• substate.touched_addresses — EIP-161 cleanup set
• frame.refund — the gas refund counter. A child starts at 0, so on success its (possibly negative) counter is added to the parent's; on failure it is dropped.

The tracer field is not revertible — trace events accumulate monotonically and are always preserved across child boundaries regardless of the child's outcome.

What this module does NOT cover

• Stack and memory: those are local to a frame and never propagate child → parent on success or failure.
• Gas accounting: returning the child's unused child.gas to the parent happens at the call site, not here, because the math depends on opcode-specific bookkeeping (e.g. CREATE's deposit cost).
• The :db override in CREATE: the success path replaces child.db with a post-deploy db that has the runtime code installed. Call sites apply that override after merge_child_result/2.