feat: add clap CLI arg parser, replacing MANDEL_* debug env vars

Prep work for a headless render mode: native debug flags (kind, power,
julia, share, view, de, buddhabrot, export, ...) are now parsed with
clap instead of read from env vars, with proper --help/--version.

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

src/main.rs

@@ -9,10 +9,12 @@
 // and calls the wasm `main`, which boots eframe onto the page's <canvas>.
 
 mod app;
-mod lights;
 mod fractal;
+mod lights;
 mod view;
 
+#[cfg(not(target_arch = "wasm32"))]
+mod cli;
 #[cfg(not(target_arch = "wasm32"))]
 mod worker;