docs(cache): document contract-cache layer and add-a-reader recipe

[Tobi-8]

Description

The frontend's contract cache layer (lib/cache/contract-cache.ts) and registry required clear documentation to prevent misuse (e.g., caching write operations or post-transfer balances, or setting improper TTLs) and help contributors confidently add new cached reads.

This PR introduces comprehensive documentation for the caching subsystem without altering its core behavior.

Changes Made

• TSDoc Added: Documented public functions across lib/cache/contract-cache.ts and lib/cache/registry.ts detailing parameters, invalidation, safety protocols, and TTL semantics.
• Cache Guide Created: Added docs/contract-cache.md which thoroughly explains:
  • The read/cache/invalidate lifecycle.
  • The manual invalidation API endpoint.
  • A step-by-step recipe for safely adding a new cached read.
  • Critical edge cases (e.g., what must never be cached).
• README Cross-link: Linked the new caching guide under the Project Structure section in the main README.md.
• Test Fixes: Fixed a bug in tests/unit/cached-wrappers.test.ts where mock Stellar addresses were 58 characters long. They have been updated to 56 characters so they validate seamlessly against the STELLAR_ADDRESS_REGEX.

Verification

• All unit tests (including conceptual cache tests) pass locally (npm run test:unit).
• Linter passes on all modified files without warnings (npm run lint).

closes #477