feat(lean4): add formal verification specs for ensemble models

Lean 4 formalization of the decision tree + MLP ensemble architecture.
Axiomatizes Float properties (sigmoid bounds, ReLU nonnegativity) since
Lean's Float ops are extern-backed. Proves MLP output is bounded in (0,1)
and ensemble output is always a valid decision. No mathlib dependency.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

lean4/lakefile.lean

@@ -0,0 +1,11 @@
+import Lake
+open Lake DSL
+
+package «sunbeam» where
+  leanOptions := #[
+    ⟨`autoImplicit, false⟩
+  ]
+
+@[default_target]
+lean_lib «Sunbeam» where
+  srcDir := "."