sync OilVinegar.lean from PR #19 (28 theorems, div_lt_div_iff deprecation fix)

v3: 28 theorems — add §7 post-quantum modular + §8 quantum-resilient hardness

v2: honest framing — uniqueness theorem, not a cryptosystem

fix: sync OilVinegar.lean with merged GitHub version (18 theorems)

fix: align model card with merged OilVinegar.lean (18 theorems)

fix: add YAML metadata to repo card

feat: μ-OV cipher — post-quantum signatures from the Eigenverse

initial commit