perf: add SignerContext caching for ring signature optimization

[jorgecuesta]

Summary

• Add signerContextCache (sync.Map) to Signer struct to cache SignerContext instances per ring
• Implement getOrCreateSignerContext() for thread-safe cache access
• Update Sign() to use ring-go's SignWithContext() with cached context
• Add ClearSignerContextCache() for session rollover cleanup
• Update ring-go dependency to include SignerContext optimization (feat: add SignerContext for optimized repeated signing ring-go#3)

Problem

Ring signatures consume significant CPU time when signing relay requests. The existing implementation recomputes expensive cryptographic operations (ScalarBaseMul, hashToCurve, key image) on every sign call, even when using the same ring within a session.

Solution

Cache the pre-computed SignerContext per ring. Since rings are tied to sessions (same sessionEndHeight = same ring), this cache is highly effective for the typical use case of signing multiple requests within the same session.

Benchmark Results (ring-go)

Ring Size	Improvement
2	~17.4% faster
4	~12.6% faster
8	~5.7% faster

Usage Note

Callers should invoke ClearSignerContextCache() on session rollover to prevent unbounded cache growth.

Test Plan

• All existing tests pass
• Build succeeds
• Benchmarks verify performance improvement