feat: preflight Fusion readiness

[steipete]

Summary

• preflight an unsaved Fusion profile against a selected policy
• report per-call route, grant, verification, pricing, and budget readiness
• show eligible-call reservation estimates and remaining policy budget
• block enabling a profile when its synthesizer cannot pass deterministic preflight

Verification

• pnpm check
• pnpm build
• pnpm worker:e2e
• git diff --check
• autoreview: clean after fixing all accepted findings

Behavior covered

• malformed, missing, and unknown policies
• provider-denied and executable profiles
• exhausted, underfunded, zero-cost, fixed-price, and unpriced budget cases
• worst-case adviser JSON expansion
• stale asynchronous previews and independent policy selection