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

Cargo.toml

@@ -13,6 +13,7 @@ png = "0.18.1"
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 env_logger = "0.11.11"
+clap = { version = "4.5.51", features = ["derive"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 futures-channel = { version = "0.3.34", default-features = false, features = ["alloc", "std"] }