Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/github.com/go-chi/chi/v5/middleware/content_charset.go

@@ -0,0 +1,45 @@
+package middleware
+
+import (
+	"net/http"
+	"slices"
+	"strings"
+)
+
+// ContentCharset generates a handler that writes a 415 Unsupported Media Type response if none of the charsets match.
+// An empty charset will allow requests with no Content-Type header or no specified charset.
+func ContentCharset(charsets ...string) func(next http.Handler) http.Handler {
+	for i, c := range charsets {
+		charsets[i] = strings.ToLower(c)
+	}
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if !contentEncoding(r.Header.Get("Content-Type"), charsets...) {
+				w.WriteHeader(http.StatusUnsupportedMediaType)
+				return
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+// Check the content encoding against a list of acceptable values.
+func contentEncoding(ce string, charsets ...string) bool {
+	_, ce = split(strings.ToLower(ce), ";")
+	_, ce = split(ce, "charset=")
+	ce, _ = split(ce, ";")
+	return slices.Contains(charsets, ce)
+}
+
+// Split a string in two parts, cleaning any whitespace.
+func split(str, sep string) (string, string) {
+	a, b, found := strings.Cut(str, sep)
+	a = strings.TrimSpace(a)
+	if found {
+		b = strings.TrimSpace(b)
+	}
+
+	return a, b
+}