feat: add SignerContext for optimized repeated signing

[jorgecuesta]

Summary

Add SignerContext struct and SignWithContext method to pre-compute cryptographic values that remain constant across multiple sign operations with the same private key:

• Public key (G * privKey)
• Hash-to-curve result (H_p(pubkey))
• Key image (privKey * H_p(pubkey))
• Signer index in the ring

Performance Results

Ring Size	Sign	SignWithContext	Improvement
2	1.02ms	0.84ms	17.4%
4	1.77ms	1.54ms	12.6%
8	3.20ms	3.02ms	5.7%
16	6.00ms	5.85ms	2.5%

Use Case

This optimization is particularly beneficial for gateway/relay systems (like PATH) that sign many requests with the same key during a session. The SignerContext can be created once when a session starts and reused for all subsequent sign operations.

Usage

// Create context once (e.g., at session start)
ctx, err := ring.NewSignerContext(privKey)

// Reuse for multiple sign operations
sig1, _ := ring.SignWithContext(msg1, ctx)
sig2, _ := ring.SignWithContext(msg2, ctx)  // much faster

Test plan

• All existing tests pass
• New tests for SignWithContext (validity, linkability, error cases)
• Benchmarks show improvement