M12.3: tracker core

New L2 crate effectcraft-track: per-point feature/search region tracking with
pyramid normalized cross-correlation and Lucas-Kanade (similarity + gain/bias)
sub-pixel refinement, confidence, adapt/stop/extrapolate/continue actions,
RGB/luminance/saturation channels, blur/enhance pre-processing, and
homography/affine/similarity solves.

Project: Motion Trackers > Tracker n > Track Point n property tree
(GroupKind::Tracker with type, target and options). Engine: track.* commands
(new/motion/stabilize/property/select/setType/setTarget/options/setPoint/
analyze/stop/apply/reset/delete/status), background or blocking analysis
job with progress and cancel (one undo step), apply to Transform, Stabilize
and Corner Pin. Renderer::layer_source for the tracker's frames.

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

crates/render/src/lib.rs

@@ -329,6 +329,13 @@ impl<'a> Renderer<'a> {
         Some(flat)
     }
 
+    /// A layer's source pixels at the context time, before masks, effects and the transform
+    /// (what the motion tracker analyses, like After Effects' Layer panel). `None` for layers
+    /// without pixels.
+    pub fn layer_source(&self, ctx: &EvalCtx, layer: &Layer) -> Option<Buf> {
+        self.source(ctx, layer)
+    }
+
     /// Layer pixels after source → masks → effects (no layer styles).
     pub fn content_buf(&self, ctx: &EvalCtx, layer: &Layer) -> Option<Arc<Buf>> {
         self.content_buf_timed(ctx, layer, None).map(|(b, _)| b)