Add contract read cache with smart invalidation

[Just-Bamford]

Problem

Contract reads are cached but no smart invalidation when state changes. Stale data served until TTL expires.

Solution

Implement smart cache invalidation in
eadContract(). Track contract state changes. Invalidate cache on modifications.

Acceptance Criteria

• Smart cache added to
  eadContract()\
• Tracks state changes for invalidation
• Invalidates on contract modifications
• Tests cover invalidation scenarios

Note for contributors

Reference contract events for change detection. Add tests to \src/tests/soroban.test.ts.