feat(core): enable OpenRouter Claude prompt caching

[dexhunter]

Summary

• enable prompt caching by default for Claude models on the official OpenRouter API endpoint, with promptCaching: false as an opt-out
• forward BAML cache_control role metadata without changing prompt content or the existing four-breakpoint limit
• report OpenRouter uncached, cache-read, and cache-write input tokens while preserving native Anthropic accounting and cumulative fallbacks

Fixes #80.

Performance

A deterministic five-turn prompt-cache harness reduced modeled uncached prompt tokens from 11,250 to 4,000 (64.4%). It verifies identical prompt content and order, cache-breakpoint limits, non-Claude and non-OpenRouter controls, endpoint spoofing controls, malformed usage clamping, native Anthropic behavior, and multi-model opt-out ordering before reporting the metric.

The direction was found by running autoresearch with Weco, then rebuilt as a focused patch and validated independently: public trajectory.

Validation

• bun test packages/magnitude-core/src --timeout 15000 (46 passed)
• cd packages/magnitude-core && bun run check
• bun run build
• strict evaluator: prompt_cache_uncached_input_tokens=4000
• git diff --check