Deprecate router v1

Context

The legacy router has accumulated 4 years of rule-DSL extensions. p95 ~800ms. Tested upper bound on optimization is ~500ms — not enough.

Decision

Replace, do not refactor. New service is order-routing-v2; old router enters deprecation mode at GA (target mid-June) and is fully removed by end of Q3.

Why

• p95 ceiling on the old router cannot meet our target.
• The legacy DSL doesn't model customer-authored rules — would require a rewrite anyway.
• Customers need migration time; deprecation window is generous (3 months) on purpose.

Consequences

• All three design partners (acme-logistics, northwind-supply, stellar-freight) will be migrated.
• Self-hosted edge (Northwind ask) is not carried forward in v2 — see northwind-supply for the stopgap.
• Engineering owns the cutover; CS owns customer comms.