Enrich binary thresholding: tunable fixed + Bradley–Roth adaptive

BinaryFrontend gains a Threshold enum: Fixed(u8) (now tunable, was
hardcoded to 128) and Adaptive { window, t } — Bradley–Roth adaptive
thresholding computed via visioncortex's SummedAreaTable, O(pixels)
regardless of window size, for images with uneven lighting. Both use a
shared (r+g+b)/3 intensity so they agree on "dark"; the grayscale
checker_bw golden is unaffected.

Exposed through Config and all bindings: CLI (--threshold, --adaptive,
--adaptive-window, --adaptive-t), Python (constructor kwargs + getters/
setters), and the Node package (binaryThreshold, adaptive, adaptiveWindow,
adaptiveT). Adds tests covering fixed tunability and adaptive recovering
locally-dark marks under a brightness gradient that a global cutoff can't.

crates/vtracer-cli/src/main.rs

@@ -90,6 +90,22 @@ struct Args {
     /// Optimization level: 0 = off, 1 = quantize+simplify, 2 = + shorthands/grouping.
     #[arg(long, value_parser = clap::value_parser!(u8).range(0..=2))]
     optimize: Option<u8>,
+
+    /// Binary mode: fixed threshold (0..=255); foreground when intensity is below it.
+    #[arg(long, value_parser = clap::value_parser!(u8))]
+    threshold: Option<u8>,
+
+    /// Binary mode: use Bradley–Roth adaptive thresholding (handles uneven lighting).
+    #[arg(long)]
+    adaptive: bool,
+
+    /// Adaptive window side length in px (0 = auto). Implies --adaptive.
+    #[arg(long)]
+    adaptive_window: Option<u32>,
+
+    /// Adaptive sensitivity: percent below the local mean (default 15). Implies --adaptive.
+    #[arg(long)]
+    adaptive_t: Option<f64>,
 }
 
 fn parse_segment_length(s: &str) -> Result<f64, String> {
@@ -169,6 +185,21 @@ fn build_config(args: &Args) -> Result<Config, String> {
         config.max_colors = Some(v);
     }
 
+    // Binary thresholding: --adaptive (or either adaptive tuning flag) selects
+    // Bradley–Roth; otherwise --threshold tunes the fixed cutoff.
+    if let Some(v) = args.threshold {
+        config.binary_threshold = v;
+    }
+    if args.adaptive || args.adaptive_window.is_some() || args.adaptive_t.is_some() {
+        config.binary_adaptive = true;
+    }
+    if let Some(v) = args.adaptive_window {
+        config.binary_adaptive_window = v;
+    }
+    if let Some(v) = args.adaptive_t {
+        config.binary_adaptive_t = v;
+    }
+
     // Palette: inline flag wins over file; both parse to a color list.
     if let Some(text) = &args.palette {
         config.palette = parse_palette(text)?;