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

Cargo.toml

@@ -14,6 +14,7 @@ png = "0.18.1"
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 env_logger = "0.11.11"
 clap = { version = "4.5.51", features = ["derive"] }
+pollster = "1.0.1"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 futures-channel = { version = "0.3.34", default-features = false, features = ["alloc", "std"] }