- Every block now emits a C label (its id) before its code
- generateNextCode emits goto label; for back-edges (visited targets)
- _emitBranch delegates to generateCode (not generateOwnCode), so labels
are emitted consistently in both main and branch traversals
- generateCallWithError threads a shared visited set across all three
emitBranch calls, preventing duplicate label emission for shared blocks
- emitBranch skips roots already visited by previous roots in the same
branch (prevents spurious gotos from shared-block traversal)