Brummel
0419e54987
Three tasks, each leaving the workspace green: 1. Sub 2-input difference node in aura-std (+ test). 2. Sim in aura-engine: Edge/Target/BootstrapError + bootstrap (schema-sizing, edge kind-check, Kahn topo-sort, cycle/bad-index/kind-mismatch rejection) + the forwarding run loop (disjoint &mut-self destructure, no per-cycle alloc), with 5 tests (chain, fan-out/join + determinism, cycle, kind-mismatch, bad-index). aura-engine gains a test-only dev-dependency on aura-std. 3. Workspace gate: build / test (26) / clippy -D warnings / purity grep. Orchestrator decisions baked in: observe captured at eval-time (no output column); Sim stores only fields run reads (no write-only cycle_id/source_kind that would trip `field never read`); the sim.rs doc avoids the literal Rc/RefCell tokens so the purity grep does not self-match.