fix(circomgnark): cache verifying key conversions

[altergui]

No description provided.

[Copilot]

Pull request overview

This pull request implements caching for verification key conversions in the circomgnark package to optimize performance when the same verification keys are converted multiple times. The PR moves verification key conversion to occur only when needed (when fixedVk is false) in the ToGnarkRecursion function, and adds two separate caching layers: one for unmarshalling CircomVerificationKey from JSON and another for converting CircomVerificationKey to Gnark format.

Changes:

• Moved verification key conversion in ToGnarkRecursion from unconditional to conditional (only when fixedVk is false) to avoid unnecessary work
• Added caching layer in UnmarshalCircomVerificationKeyJSON to return the same parsed object for identical JSON input
• Added caching layer in CircomVerificationKey.ToGnark() to return the same converted verification key for the same input object
• Introduced comprehensive tests to verify caching behavior and the optimization

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
util/circomgnark/recursion.go	Moved verification key conversion to occur only when fixedVk is false
util/circomgnark/conv.go	Added caching to ToGnark() method using pointer-based cache keys
util/circomgnark/marshal.go	Added caching to UnmarshalCircomVerificationKeyJSON() using string-based cache keys
util/circomgnark/cache_test.go	Added tests to verify caching behavior

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