secp256k1: add constants for compact signature size

[MichaelMure]

Note

Low Risk
Low risk: adds a size constant and uses it for input validation/error messages without changing signature recovery behavior.

Overview
Adds SignatureCompactBytesSize (65 bytes) to the secp256k1 crypto package and documents the meaning of “compact” signatures.

Updates PublicKeyFromRecovery to validate compact signature length against this constant and to use it in the error message, replacing the previous hard-coded 65.

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