feat: scaffold vector and raster component pipeline

[AsPulse]

Initial scaffolding of tellur's component pipeline.

• VectorComponent and RasterComponent traits, each with
  view_box() -> Vec2 and a render() method; pixel resolution
  flows through RasterComponent::render(target: Resolution).
• tellur-core::vector: VectorGraphic, Node (Group / Path),
  PathCommand, Fill, Stroke, Paint.
• tellur-core::shapes: Rectangle, Circle, Ellipse anchored at
  local origin so positioning is a composition-time concern.
• tellur-core::geometry: Vec2, Rect, Transform, plus
  Anchor and AnchoredSize::snap_to for fractional-anchor placement.
• tellur-core::layer: VectorLayer (translate-wrapped vector
  composition) and Layer (per-child sub-resolution render +
  source-over alpha compositing).
• tellur-core::raster: RasterImage with export_png (RGBA8,
  straight alpha).
• tellur-core::color: Color with rgb_u8 / hsv / hsl (+ alpha).
• tellur-renderer: Rasterize<V> (tiny-skia based) and the
  Rasterizable ext trait so any VectorComponent can become a
  RasterComponent via .rasterize().
• Examples: scene_to_png (single-rasterize VectorLayer path) and
  raster_layer_to_png (per-child Layer path with translucent
  overlap blending).