Lean migration: Phase 4 (IterProd + FiniteMap lattices)

- Spa.Lattice.IterProd: k-fold product, recursive Lattice instance,
  fixed height k*hA + hB, bot = build of bottoms
- Spa.Lattice.FiniteMap: spine-pinned assoc lists ({l // l.map fst = ks});
  with = the 1100-line Map.agda collapses into positional 'combine'.
  Same lemma inventory (membership, locate, updating, GeneralizedUpdate,
  valuesAt, Provenance-union, le_of_mem_mem) — Nodup is now an explicit
  hypothesis where the Agda Map carried it intrinsically. Fixed height
  |ks|*hB still via transport along the IterProd isomorphism, which no
  longer needs Unique ks (representation is canonical).

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

LEAN_MIGRATION.md

@@ -74,10 +74,10 @@ validate phase by phase.
 ## Status
 
 - [x] Phase 0
-- [ ] Phase 1
-- [ ] Phase 2
-- [ ] Phase 3
-- [ ] Phase 4
+- [x] Phase 1
+- [x] Phase 2
+- [x] Phase 3
+- [x] Phase 4
 - [ ] Phase 5
 - [ ] Phase 6
 - [ ] Phase 7