WorldContextBuilder v2: all 6 tiers with parallel async assembly

[theinterneti]

Part of #266

Implement tiers 1-6 with async parallel fetching (Neo4j + Dolt + Redis simultaneously). Assembly time target: < 300ms p99. Each tier has configurable token budget.

Acceptance Criteria

• All 6 tiers implemented with async parallel data fetching
• Neo4j, Dolt, and Redis queries fire concurrently via asyncio.gather
• Assembly time < 300ms p99 measured against benchmark dataset
• Each tier's token budget configurable via settings
• Benchmark test added to CI