refactor(v0.2): вынести headless render в shturman-render (shell использует)

P6.1: общий хелпер render_to_png<C: ComponentHandle>(build, w, h, path) поверх
Slint software-renderer (thread_local окно + set_platform once + draw + png).
shturman-shell.render_screenshot теперь зовёт его; плумбинг-дубль удалён.
png в shell → dev-deps (рендер в render-крейте, тест декодирует). Тесты зелёные.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Alexander <akotenev2003@gmail.com>

Cargo.lock

@@ -3589,6 +3589,16 @@ dependencies = [
  "zbus 4.4.0",
 ]
 
+[[package]]
+name = "shturman-render"
+version = "0.0.0"
+dependencies = [
+ "anyhow",
+ "png 0.17.16",
+ "slint",
+ "tempfile",
+]
+
 [[package]]
 name = "shturman-sdk"
 version = "0.0.0"
@@ -3624,6 +3634,7 @@ dependencies = [
  "anyhow",
  "png 0.17.16",
  "shturman-common",
+ "shturman-render",
  "shturman-sdk",
  "slint",
  "tempfile",