Add production-ready HTTP server package with routing, health checks, and middleware

Introduces server/ sub-package as the server-side companion to the existing Client.
Includes Router (over http.ServeMux with groups and mounting), graceful shutdown with
signal handling, health endpoints (/healthz, /readyz), and built-in middlewares
(RequestID, Recovery, Logging). Zero external dependencies.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/middleware_requestid.go

@@ -0,0 +1,52 @@
+package server
+
+import (
+	"context"
+	"crypto/rand"
+	"fmt"
+	"net/http"
+)
+
+type requestIDKey struct{}
+
+// RequestID returns a middleware that assigns a unique request ID to each
+// request. If the incoming request already has an X-Request-Id header, that
+// value is used. Otherwise a new UUID v4 is generated via crypto/rand.
+//
+// The request ID is stored in the request context (retrieve with
+// RequestIDFromContext) and set on the response X-Request-Id header.
+func RequestID() Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			id := r.Header.Get("X-Request-Id")
+			if id == "" {
+				id = newUUID()
+			}
+
+			ctx := context.WithValue(r.Context(), requestIDKey{}, id)
+			w.Header().Set("X-Request-Id", id)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+// RequestIDFromContext returns the request ID from the context, or an empty
+// string if none is set.
+func RequestIDFromContext(ctx context.Context) string {
+	id, _ := ctx.Value(requestIDKey{}).(string)
+	return id
+}
+
+// newUUID generates a UUID v4 string using crypto/rand.
+func newUUID() string {
+	var uuid [16]byte
+	_, _ = rand.Read(uuid[:])
+
+	// Set version 4 (bits 12-15 of time_hi_and_version).
+	uuid[6] = (uuid[6] & 0x0f) | 0x40
+	// Set variant bits (10xx).
+	uuid[8] = (uuid[8] & 0x3f) | 0x80
+
+	return fmt.Sprintf("%x-%x-%x-%x-%x",
+		uuid[0:4], uuid[4:6], uuid[6:8], uuid[8:10], uuid[10:16])
+}