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/cli.rs

@@ -0,0 +1,84 @@
+// Native command-line arguments. Currently mirrors the old `MANDEL_*` debug
+// env vars one-for-one; this is the foundation a future headless (no-window,
+// render-to-file) mode will build on.
+
+use clap::{Parser, ValueEnum};
+
+use crate::fractal::FractalKind;
+
+#[derive(Parser, Debug, Default)]
+#[command(name = "mandelbrot", about = "Deep-zoom fractal explorer", version)]
+pub struct Cli {
+    /// Fractal formula to render.
+    #[arg(long, value_enum)]
+    pub kind: Option<KindArg>,
+
+    /// Exponent for the Multibrot kind (z -> z^power + c), clamped to [2, 8].
+    #[arg(long)]
+    pub power: Option<u32>,
+
+    /// Start in Julia mode with this seed constant.
+    #[arg(long, value_name = "RE,IM")]
+    pub julia: Option<String>,
+
+    /// Restore a view from a share-link fragment (the part after '#').
+    #[arg(long, value_name = "FRAGMENT")]
+    pub share: Option<String>,
+
+    /// Jump to a view on startup.
+    #[arg(long, value_name = "RE,IM,HALF_HEIGHT[,ITERATIONS]")]
+    pub view: Option<String>,
+
+    /// Enable distance-estimation shading.
+    #[arg(long)]
+    pub de: bool,
+
+    /// Switch to the Buddhabrot renderer.
+    #[arg(long)]
+    pub buddhabrot: bool,
+
+    /// Buddhabrot tonemap palette index.
+    #[arg(long, value_name = "INDEX")]
+    pub buddha_palette: Option<u32>,
+
+    /// Render a PNG export on startup.
+    #[arg(long)]
+    pub export: bool,
+
+    /// Output path for --export (default: fractal-<timestamp>.png).
+    #[arg(long, value_name = "PATH")]
+    pub export_path: Option<String>,
+}
+
+#[derive(Copy, Clone, Debug, ValueEnum)]
+pub enum KindArg {
+    Mandelbrot,
+    #[value(alias = "ship")]
+    BurningShip,
+    #[value(alias = "mandelbar")]
+    Tricorn,
+    #[value(alias = "multi")]
+    Multibrot,
+    Celtic,
+    #[value(alias = "perp")]
+    Perpendicular,
+    Buffalo,
+    Phoenix,
+    Lambda,
+}
+
+impl From<KindArg> for FractalKind {
+    fn from(k: KindArg) -> Self {
+        match k {
+            KindArg::Mandelbrot => FractalKind::Mandelbrot,
+            KindArg::BurningShip => FractalKind::BurningShip,
+            KindArg::Tricorn => FractalKind::Tricorn,
+            KindArg::Multibrot => FractalKind::Multibrot,
+            KindArg::Celtic => FractalKind::Celtic,
+            KindArg::Perpendicular => FractalKind::Perpendicular,
+            KindArg::Buffalo => FractalKind::Buffalo,
+            KindArg::Phoenix => FractalKind::Phoenix,
+            KindArg::Lambda => FractalKind::Lambda,
+        }
+    }
+}