I3: ec_mul intrinsic — point scalar multiplication for DVM

[liqdmetal]

Summary

I3 from the intrinsic agenda: ec_mul(point_hex String, scalar Uint64) -> String — point scalar multiplication for the DVM, gated >= 9.0.0, 30k gas.

What it does

Multiplies a compressed bn256 G1 point (DERO's 33-byte encoding) by a uint64 scalar, returning the compressed result. The homomorphic counterpart to ec_add (I2, already in DVM v9 PR #84):

Op	Property
ec_add(c1, c2)	c1 + c2 (commitment accumulation)
ec_mul(c, k)	k·c (point derivation, key blinding, commitment scaling)

Together they give a contract complete in-VM group arithmetic on commitments — point derivation for key blinding, commitment scaling, and accumulation — without oracle dependence.

Tests (TestEcMul)

• ec_mul(c, 2) == ec_add(c, c) — the homomorphic pair
• scalar composition: ec_mul(ec_mul(c, 3), 2) == ec_mul(c, 6)
• ec_mul(c, 1) == c (identity)
• ec_mul(c, 0) produces a valid point encoding
• output is always a 33-byte compressed point (66 hex)
• version gate: invisible to 1.2.3 contracts

Relationship

• Stacks on PR DVM v9 intrinsics: signature verification, Pedersen commitments, asset balance, point addition (fixes #83) #84 (DVM v9 intrinsics) — same >= 9.0.0 gate, same group-arithmetic family
• Completes the ec_add/ec_mul pair; the AMM-reserve/confidential-settlement use case is now fully expressible in-VM

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Branch: feature/dvm-i3-ecmul in the fork liqdmetal/derohe-improvements-by-liqdmetal (stacked on feature/dvm-v9-intrinsics).