feat: add outline raster component

[AsPulse]

Adds an Outline raster effect that wraps a RasterComponent and paints a solid-colored ring around the outside of the child's alpha shape. Mirrors DropShadow in shape: it owns a width (logical units) and a color, expands paint_bounds so the stroke isn't clipped, leaves layout_box untouched, and routes the child render through ctx.render(...) so the inner subtree is memoized independently.

• tellur-renderer::outline (new): Outline { width, color, child }. paint_bounds widens the child rect by width on all sides; layout is pass-through. render rasterizes the child once, dilates the alpha channel by width, subtracts the original alpha to leave only the ring, applies color, and composites outline then child source-over.
• Dilation uses an axis-aligned ellipse SE rather than a separable square (Chebyshev) SE. A square SE is cheaper but flattens curved tips — the top/bottom of a circle becomes a horizontal cap — which is visually distracting on rounded shapes. The ellipse keeps the contour following the original alpha, including under anisotropic pixel ratios.
• Dilation radii along x and y are computed independently as round(width * sx) / round(width * sy), so the output image is exactly the size of paint_bounds * (sx, sy). Using max(sx, sy) (matching the existing DropShadow pattern) would have pushed the outline past the buffer edge under anisotropic ratios and gotten silently clipped by composite_at.
• make_outline's ring = saturating_sub(dilated, original), so pixels fully inside the child contribute zero and only the outside band carries color. color.a multiplies through, matching DropShadow's convention.
• Example: timeline_to_mp4's BouncingDot is now Circle → Outline (white, 4px) → DropShadow, so the shadow falls behind the combined stroked shape. Added an unquantized first track (raw t) ahead of the 60/30/24/16 fps tracks so the smooth motion is the explicit baseline.
• Outline is registered on tellur-renderer::lib and re-exported alongside DropShadow. CachingRenderContext picks it up automatically — in the example the entire Outline subtree is one miss + 1199 hits across the timeline.