Function createZKRangeProof

Prove value >= threshold without revealing value.

Method: Prove delta = value - threshold >= 0 by decomposing delta into bits and proving each bit is 0 or 1 using OR-proofs (Sigma protocol).

Steps:

1. Commit to delta = value - threshold
2. Decompose delta into bits: delta = Σ 2^i · b_i
3. Commit to each bit: C_i = g^{b_i} · h^{r_i}
4. Prove each C_i commits to 0 or 1 (OR-proof)
5. Prove Σ 2^i · C_i = C_delta (using homomorphic property)