// mk-core: plan9 mk build-tool core library.
//
// Architecture:
// mkfile text → lex → tokens → parse → AST → graph → DAG → sched → recipe exec
//
// Modules:
// lex — tokenizer (F-001..F-016)
// attr — rule attribute bitflags (F-009..F-010, F-023..F-028)
// error — unified error type (all modules)
// parse — parser: rules, assignments, includes (Phase 1b)
// graph — DAG builder, staleness, cycle detection (Phase 1a)
// var — variable system, symbol table, expansion (Phase 1a)
// shell — Shell trait (Phase 1a)
// recipe — recipe execution glue (Phase 1a)
// sched — scheduler: serial & parallel execution (Phase 1a serial, Phase 2 parallel)
// include — recursive mkfile includes (Phase 1b)
// archive — lib(member) syntax (Phase 3)