Visitor-hash collapse when IP+UA are 'unknown'

Severity: High · Where: dynamodb.ts:287, collect.ts:118; fallbacks lambda-adapter.ts:204,211 When both IP and UA are 'unknown' (Lambda fallbacks), all such visitors hash identically. Impact: Unique-visitor inflation/deflation; an attacker can suppress visibility by omitting the User-Agent. Fix: Treat unknown IP/UA as non-identifying (skip or per-request salt) rather than a shared constant.