feat: add contraction kernels for exterior transport

FCAP/Mathlib/LinearAlgebra/CliffordAlgebra/Contraction.lean

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+/-
+Copyright (c) 2026 utensil. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+-/
+module
+
+public import Mathlib.LinearAlgebra.CliffordAlgebra.Contraction
+
+/-!
+# Contraction kernels for exterior transport
+
+The change-of-form construction behind `equivExterior` contains a contraction
+correction. This file isolates a vanishing criterion for that correction and
+its two-generator orthogonal instance. It makes no claim about arbitrary
+products or a multiplicative equivalence.
+-/
+
+@[expose] public section
+
+namespace CliffordAlgebra
+
+universe uR uM
+
+variable {R : Type uR} {M : Type uM}
+variable [CommRing R] [AddCommGroup M] [Module R M]
+variable [Invertible (2 : R)]
+
+/-- The exterior equivalence transports a product of orthogonal generators to
+the corresponding product for the zero quadratic form. -/
+theorem equivExterior_ι_mul_ι_of_isOrtho (Q : QuadraticForm R M) {m n : M}
+    (h : Q.IsOrtho m n) :
+    equivExterior Q (ι Q m * ι Q n) =
+      ι (0 : QuadraticForm R M) m * ι (0 : QuadraticForm R M) n := by
+  rw [equivExterior, changeFormEquiv_apply, changeForm_ι_mul_ι]
+  simp [QuadraticMap.associated_isOrtho.mpr h]
+
+omit [Invertible (2 : R)] in
+/-- Left contraction by a dual vector vanishes on an ordered generator product
+when it vanishes on every generator in the list. -/
+theorem contractLeft_list_prod_ι_of_forall_eq_zero
+    (Q : QuadraticForm R M) (d : Module.Dual R M) (l : List M)
+    (h : ∀ m ∈ l, d m = 0) :
+    contractLeft (Q := Q) d ((l.map (ι Q)).prod) = 0 := by
+  induction l with
+  | nil => simp
+  | cons m l ih =>
+    simp only [List.map_cons, List.prod_cons]
+    rw [contractLeft_ι_mul, h m (by simp)]
+    simp [ih (fun n hn ↦ h n (by simp [hn]))]
+
+end CliffordAlgebra