Add GPT-5.6 model support

[Pimpmuckl]

Summary

• Add GPT-5.6 Sol, Terra, and Luna model normalization aliases and dashboard colors.
• Add GPT-5.6 pricing, including separate cache-write rates.
• Parse Responses usage cache-read/cache-write details and keep cache-write tokens out of ordinary uncached input billing.
• Preserve max reasoning effort in model breakdowns and live summaries.

Validation

• npm test passed: 75 tests.
• git diff --check passed.
• Review Suite emergency passed: rvw_9e7c754d.

Notes

• Existing Vite large chunk warning remains unchanged by this support pass.