M21-P4: Cache usage metrics tracking

[bug-ops]

Parent: #336

Summary

Parse Anthropic usage response fields to track cache effectiveness.

Requirements

1. Parse usage block from Claude API responses (both streaming and non-streaming)
2. Track cache_read_input_tokens and cache_creation_input_tokens
3. Add fields to MetricsSnapshot and expose via TUI/logs

Acceptance Criteria

• Cache hit/miss tokens visible in metrics
• Logging shows cache effectiveness per request

Files

• crates/zeph-llm/src/claude.rs
• crates/zeph-core/src/metrics.rs