Smaller API/protocol niceties

[FumingPower3925]

Grab-bag of smaller gaps, each independently shippable:

• Host-based / virtual-host routing (router keys on method+path only today)
• Typed route-parameter constraints (:id currently routes then errors)
• Early Hints (103) / interim 1xx before the final status
• Additional response serializers (YAML, MsgPack, JSONP)
• i18n / localization middleware (Accept-Language message catalog)