sdk: add getRound caching with explicit invalidation

[Ugooweb]

Closes #158

Summary

Provides an optional short-lived cache for getRound and getConfig reads in SubRosaClient, paired with explicit cache invalidation on write operations (e.g. commit, reveal, settle).

Motivation

Dashboards and keeper watch loops frequently re-read the same round state and configuration repeatedly. Previously, these uncached reads added avoidable RPC load and latency. This PR introduces an opt-in TTL-based cache to reduce overhead for consumers making heavy sequential reads while maintaining consistency guarantees by invalidating immediately upon localized state changes.

Changes

• cacheTtl configuration option: Added a cacheTtl property (in milliseconds) to SubRosaClientConfig. It defaults to 0 (disabled) to remain backwards compatible and avoid stale reads by default.
• getRound & getConfig caching: Implemented a lightweight in-memory cache map for getRound results indexed by roundId, and a single cache slot for getConfig.
• Explicit Invalidation: Added #invalidateRound(roundId) which wipes the cache entry for a specific round. This is now called after any successfully submitted state-changing write operations related to a round, specifically: commit, openReveal, reveal, clear, settle, and void.
• Testing: Added unit tests in client.test.ts to ensure that caching returns results properly without incrementing underlying contract calls and that cache is appropriately invalidated (and fresh reads are fetched) upon write.
• Documentation: Updated the SDK README.md to document the tradeoffs of using cacheTtl, ensuring developers are aware of how

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