[FEATURE]: Implement Lazy Hashing for Transactions

[sanaica]

Description

Currently, the Transaction.tx_id calculation logic re-computes the SHA-256 hash every time the identifier is requested. During high-load operations like block validation or full-node syncing, the CPU performs these expensive cryptographic operations thousands of times for the same static data.

By implementing a caching pattern, we store the result of the first hash calculation and reuse it for all subsequent calls, significantly reducing redundant CPU cycles.

Proposed Optimization

1. Lazy Initialization: Add a _cached_tx_id attribute to the Transaction class (initialized as None in __init__).
2. Memoization: Update the @property tx_id to check if _cached_tx_id exists. If not, it calculates the hash once via canonical_json_hash and stores it.
3. Cache Invalidation: The sign() method modifies the transaction payload by adding a signature, will reset _cached_tx_id to None to ensure the ID always reflects a signed state.

Expected Impact

• Performance: Dramatically reduces SHA-256 overhead during validation loops.
• Minimality: Implemented with a code footprint, maintaining the project's "strictly minimal" philosophy.
• Scalability: Complements the ongoing $O(1)$ mempool work by ensuring ID retrieval is as fast as the dictionary lookup.

Roadmap Alignment

This supports the "Core Perfection" phase to make it minimal, clean, and performant.