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.

nodejs/src/lib.rs

@@ -29,6 +29,14 @@ struct Options {
     palette: Option<Vec<String>>,
     max_colors: Option<usize>,
     optimize: Option<u8>,
+    /// Binary-mode fixed threshold (0..=255).
+    binary_threshold: Option<u8>,
+    /// Binary mode: use Bradley–Roth adaptive thresholding.
+    adaptive: Option<bool>,
+    /// Adaptive window side length in px (0 = auto).
+    adaptive_window: Option<u32>,
+    /// Adaptive sensitivity: percent below the local mean (default 15).
+    adaptive_t: Option<f64>,
     /// One of "bw" | "poster" | "photo"; applied before the other fields.
     preset: Option<String>,
 }
@@ -105,6 +113,19 @@ fn config_from(options: JsValue) -> Result<Config, JsValue> {
     if let Some(v) = opts.optimize {
         config.optimize = v;
     }
+    if let Some(v) = opts.binary_threshold {
+        config.binary_threshold = v;
+    }
+    // Any adaptive tuning field (or `adaptive: true`) switches on Bradley–Roth.
+    if opts.adaptive == Some(true) || opts.adaptive_window.is_some() || opts.adaptive_t.is_some() {
+        config.binary_adaptive = true;
+    }
+    if let Some(v) = opts.adaptive_window {
+        config.binary_adaptive_window = v;
+    }
+    if let Some(v) = opts.adaptive_t {
+        config.binary_adaptive_t = v;
+    }
     Ok(config)
 }