feat: add a headless mode, driven by clap CLI args

--headless renders the current view straight to a PNG and exits, with
no window/event loop: it builds its own offscreen wgpu device and reuses
the same reference-orbit + ExportRender path as the "Export PNG" button.
--width/--height set the output resolution (default 1920x1080); not yet
supported with --buddhabrot.

Refactors FractalApp::new's state construction and CLI handling into
default_state()/apply_cli() so headless mode can reuse them, and factors
the PNG tile-render/readback/encode loop into a shared
export_to_png_blocking() used by both the windowed export path and
headless.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018GFBP9qrNg6NGJWzzP36xe

src/main.rs

@@ -16,6 +16,8 @@ mod view;
 #[cfg(not(target_arch = "wasm32"))]
 mod cli;
 #[cfg(not(target_arch = "wasm32"))]
+mod headless;
+#[cfg(not(target_arch = "wasm32"))]
 mod worker;
 
 use app::FractalApp;
@@ -49,11 +51,24 @@ fn wgpu_options() -> eframe::egui_wgpu::WgpuConfiguration {
 
 #[cfg(not(target_arch = "wasm32"))]
 fn main() -> eframe::Result {
+    use clap::Parser as _;
+
     env_logger::builder()
         .filter_level(log::LevelFilter::Info)
         .parse_default_env()
         .init();
 
+    let cli = cli::Cli::parse();
+    if cli.headless {
+        return match headless::run(cli) {
+            Ok(()) => Ok(()),
+            Err(e) => {
+                eprintln!("error: {e}");
+                std::process::exit(1);
+            }
+        };
+    }
+
     let native_options = eframe::NativeOptions {
         renderer: eframe::Renderer::Wgpu,
         wgpu_options: wgpu_options(),