Cover the zero boundary in binomial log-concavity - #7
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The TeX statement cor.lgv.binom-unimod quantifies over all natural k and uses the standard zero-extension convention for binomial coefficients, so the k = 0 case has C(n, -1) = 0. The existing Lean theorem required 1 ≤ k because Nat subtraction truncates 0 - 1 to 0. This PR preserves the existing positive-index argument as binom_log_concave_of_pos and adds the boundary-inclusive binom_log_concave theorem with the zero extension made explicit. The new wrapper handles k = 0 by simplification and otherwise invokes the existing proof. Validation: the targeted LGV1 file compiles; the full lake build completes successfully (8078 jobs); #print axioms reports only propext, Classical.choice, and Quot.sound; no sorry is introduced.