feat(twistededwards): add fixed-base base scalar mul for eddsa

[peter941221]

Summary

• add generator-driven fixed-base affine base-point scalar multiplication for twisted Edwards curves
• use 4-bit windows with constant-time full-table scans from fixed-size scalar bytes
• wire the new path into EdDSA GenerateKey, Sign, and [S]Base inside Verify without changing generic variable-base semantics

Validation

• targeted Go tests passed across all generated twisted Edwards / EdDSA packages
• added differential fixed-base vs generic tests plus EdDSA generic-reference regressions
• local benchmarks show fixed-base affine base multiplication is consistently faster than the generic base path across the touched curves
• local dudect-like timing checks passed on representative curves; this is timing evidence, not a formal proof

Ref #775.

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Note

Medium Risk
Touches core elliptic-curve scalar multiplication and EdDSA keygen/sign/verify paths across multiple curves; any bug could break signature validity or introduce subtle side-channel/constant-time regressions despite added differential tests.

Overview
Adds a generator-driven, constant-time fixed-base scalar multiplication path for twisted Edwards curves via PointAffine.ScalarMultiplicationBase, using a lazily-initialized 4-bit window table and constant-time point selection.

Updates EdDSA implementations across supported curves to use the new fixed-base routine for public-key derivation in GenerateKey, computing R = [r]Base in Sign, and the [S]Base term in Verify.

Expands test coverage with generic-reference regressions and fixed-base vs generic differential checks, and adds benchmarks for keygen/sign plus generic-vs-fixed base multiplication performance comparisons.

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