S2.69: several objects in one Roto Brush instance, tracked in one SAM 2 pass

Strokes carry an object (0 for older projects, nothing written for 0). The
tracker follows every object in one pass and the mattes are united; the
confidence of a frame is the least sure object's. Object picker in the Layer
panel's bar; roto.options/stroke/clearStrokes/status take or report objects.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/analysis/roto.rs

@@ -51,6 +51,9 @@ pub struct RotoOptions {
     pub boundary_color: [f64; 3],
     /// Start propagating in the background after each stroke.
     pub auto_propagate: bool,
+    /// The object new strokes belong to (0 is the first). With a tracker every object is followed on its own
+    /// and their mattes are united; the classic engine treats all foreground strokes as one object.
+    pub object: u32,
 }
 
 impl Default for RotoOptions {
@@ -63,10 +66,14 @@ impl Default for RotoOptions {
             overlay_opacity: 50.0,
             boundary_color: [1.0, 0.25, 0.75],
             auto_propagate: true,
+            object: 0,
         }
     }
 }
 
+/// The highest object id (SAM 2 follows up to 32 objects in one pass).
+pub const MAX_OBJECT_ID: u32 = 31;
+
 pub const VIEWS: [&str; 4] = ["alphaBoundary", "alpha", "alphaOverlay", "none"];
 
 /// What a job does.