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Proves the split_left_le bounds lemma in Fad/Chapter5-Ex.lean.
The proof establishes a per-step invariant: each application of split₁.op increases the combined lengths of the two output lists by exactly one. By induction over foldr, the combined output length is bounded by the accumulator length plus the input length; specializing to the initial (x, [], []) accumulator gives (split₁ xs).2.1.length ≤ xs.length.
Verified with lake build Fad.«Chapter5-Ex» under both Lean v4.33.0-rc1 (repo toolchain) and v4.35.0-rc2 — builds succeed, only the pre-existing unrelated sorry remains.
Addresses #76.