Define Voicepath routing contract

[rogerchappel]

Summary

• Defines the Wave 1 Voicepath routing contract and utterance/segment lifecycle.
• Adds importable TypeScript contract types for provider capabilities, routing requests, latency budgets, lifecycle events, fallback reasons/outcomes, and continuity-critical audio chunks.
• Adds lightweight contract tests and a repo-level validation script.

Product invariants covered

• Fast first audio is represented in explicit latency budgets and fallback reasons.
• Graceful fallback is constrained to normalized reasons/outcomes and safe boundaries.
• Voice/provider continuity is locked per sentence so fallback cannot randomly switch voice mid-sentence.

Validation

• bash scripts/validate.sh
  • npm test
  • npm run typecheck
  • npm run build

Note: docs/ORCHESTRATION.md and docs/TASKS.md were not present on current main; this PR adds the requested Wave 1 contract without broadening scope beyond Voicepath.