Lean migration: Phases 0-2 (core lattice/chain, fixpoint, transport)

- lean/ lake project pinned to Lean v4.17.0 + mathlib v4.17.0
- Spa.Lattice: fold monotonicity, FixedHeight/BoundedChains (LTSeries-based),
  FiniteHeightLattice, chain-bottom-is-least; the rest of Lattice.agda,
  Chain.agda and Equivalence.agda lift into mathlib (see LEAN_MIGRATION.md)
- Spa.Fixedpoint: gas-based least-fixpoint computation (doStep/fix/aFix)
- Spa.Isomorphism: FixedHeight transport along monotone inverse pairs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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