nx_doc_demo_subject.nx

nx_doc_demo_subject.nx -- a DELIBERATELY broken subject for dogfooding the Doctor's autonomous heal on the REAL toolchain (not a synthetic KAT string). It reassigns an immutable `let` binding -> the compiler silently emits an empty .s = the #1 crash this session. nx_doc_autoheal_apply heals it (let acc -> var acc); the healed twin compiles + runs green. This file is EXPECTED to fail to build; that failure is the demo's starting state.