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.toml

@@ -9,6 +9,7 @@ members = [
     "crates/core/shturman-firstboot",
     "crates/core/shturman-settings",
     "crates/core/shturman-power",
+    "crates/apps/shturman-render",
     "crates/apps/shturman-shell",
     "crates/tools/shturman-manifest-validator",
 ]