feat: add optional Jev turn routing

[Benniphx]

Summary

• add an optional server-side Jev preflight before thread.turn.start
• support off, shadow, and confidence-gated apply modes
• fail open to the composer-selected model and record routing activities
• document environment configuration and model-tier mappings

Validation

• bun fmt
• bun lint (one pre-existing React hooks warning)
• bun typecheck
• bun run --cwd apps/server test -- jevTurnRouter.test.ts
• live synthetic TypeSafe smoke: normal -> gpt-5.6-terra, confidence 0.93

[Benniphx]

Closing this PR because it was opened against the wrong fork. The experimental Jev router will be tested on a personal fork before any upstream proposal.