add router

[KARTIKrocks]

Added

• router — New router package with route grouping and method helpers on top of http.ServeMux
• router — Method helpers: Get, Post, Put, Patch, Delete — accept func(w, r) error handlers
• router — Stdlib variants: GetFunc, PostFunc, PutFunc, PatchFunc, DeleteFunc — accept http.HandlerFunc
• router — Group(prefix, ...middleware) for prefix-based route grouping with per-group middleware
• router — Nested groups accumulate prefix and middleware (root → parent → child ordering)
• router — Handle/HandleFunc stdlib escape hatches for http.Handler/http.HandlerFunc
• router — Use() adds middleware to subsequently registered routes (matches chi/echo/gin behavior)
• router — DefaultErrorHandler writes JSON error envelope matching response.Err format, extracts *errors.Error via errors.As
• router — WithErrorHandler option for custom error handling
• router — Implements http.Handler — drop-in replacement for http.ServeMux with server.New(r)

Removed

• response — Removed Wrap function (was a no-op pass-through; use router.GetFunc/PostFunc etc. instead)