eqsat — explanations

The substrate side of the research stack — the pieces that live in handl-lang/eqsat after the plan-0032 phase-4 split.

• The e-graph and dag_cost — terms in, a saturated equivalence class out, and the cost models that rank its members.
• Certificates — Certificate/verify_certificate: the replayable derivation that is the claim.
• The ports — Source/Sink/Engine/Runner: the seam between a substrate that knows no backend and the backends that know no search.

Cross-cutting concepts (the three costs, the game, the gauntlet) live with graphopt.