Functions with if/else branches returning from both sides
produced wrong return values. The branch terminator's
loadValue(condition) DUPs the condition onto the stack;
JUMPI consumes the dup but the original remains. Successor
blocks inherit this stale value, so the return epilogue
incorrectly treated it as the return value.

Add generateReturnEpilogue that loads the return value,
cleans stale stack entries via SWAP+POP, then loads the
saved return PC and jumps back. Uses the same imperative
state pattern as generateCallTerminator.

Also changes generateTerminator return type from
Transition<S, S> to Transition<S, Stack> to accommodate
the stack-cleaning epilogue.