define_opcodes! function-like proc-macro.
Single source of truth for the VM's opcode set. One invocation in
harn-vm/src/chunk.rs emits the Op enum, the byte-to-variant mapping,
the sync and async dispatch tables, the disassembly renderer, and the
classification helpers (op_reads_outer_name, is_adaptive_binary_op).
Adding or renaming an opcode is a one-line edit; coverage drift across
the previously hand-maintained tables is now impossible.