feat: add JSON/XML/Text/Bytes/Stream/Attachment response helpers - #53

Adds six top-level response helpers that write complete HTTP responses
(headers, status code, body) on top of net/http:

• JSON(w, statusCode, data, opts...) — encodes with encoding/json;
  options: EscapeForHTML(false) to disable HTML escaping (on by default)
  and Indented(true) for pretty-printed output
• XML(w, statusCode, data) — marshals with encoding/xml, writes the
  XML declaration; marshaling errors return before any header is sent
• Text(w, statusCode, s) — text/plain; charset=utf-8 via io.WriteString
• Bytes(w, statusCode, contentType, b) — in-memory raw bytes with a
  caller-supplied content type
• Stream(w, statusCode, contentType, r) — io.Copy from any io.Reader
  with constant memory; takes the sendfile fast path for *os.File.
  Intended for large or not-yet-complete bodies (downloads, proxied
  upstream responses, generated streams); disk files that need Range and
  caching support should use http.ServeFile / http.ServeFileFS
• Attachment(w, filename, r) — sends any io.Reader as a download
  attachment with 200 OK: Content-Disposition is derived from filename
  (Unicode-safe via RFC 5987 through mime.FormatMediaType), Content-Type
  is inferred from the extension with an application/octet-stream
  fallback; built on Stream

Usage

app := sim.NewApp()

app.Get("/user", func(w http.ResponseWriter, r *http.Request) {
    if err := sim.JSON(w, http.StatusOK, user{Name: "gopher", Age: 5}); err != nil {
        slog.Error("write response", "err", err)
    }
})

// File download: Content-Disposition and Content-Type are derived
// from the filename.
app.Get("/download/{file}", func(w http.ResponseWriter, r *http.Request) {
    name := r.PathValue("file")
    f, err := os.Open(name)
    if err != nil {
        sim.JSON(w, http.StatusNotFound, map[string]string{"message": err.Error()})
        return
    }
    defer f.Close()
    sim.Attachment(w, name, f)
})