IMG-023: Buffer Reuse in Validation

[murdore]

Summary

Base64 validation decodes and re-encodes same data, wasting CPU.

Root Cause

Lines 605-620 perform redundant encode/decode operations.

Fix

Implement validation result cache using WeakMap.

Acceptance Criteria

• [ ] Implement validation result cache using WeakMap
• [ ] Cache validation results for same buffer
• [ ] Avoid redundant encode/decode operations
• [ ] Add tests for caching behavior