Boss-resolved design decisions baked in (DD-1 loop_stack in synth like
mut.2's mut_scope_stack + RecurNotInTailPosition in verify_tail_positions;
DD-2 LoopBinder derives = MutVar's, no PartialEq; DD-3 stub-then-fill
sequencing). Recon-enumerated walker-arm site list incl. the test-side
exhaustive match (codegen_import_map_fallback_pin.rs:62) mut.1's plan
missed. it.2/it.3 explicitly out of scope.
