feat: wrap layer children in Placed for inline placement

[AsPulse]

Replace the (Vec2, Box<dyn _>) tuple in layer children with a Placed<C> wrapper, and add ext traits so positioning becomes a method chain on the component itself. Layer construction can now be a single struct-literal expression with all children placed inline.

• tellur-core::placement (new): Placed<C: ?Sized> { position, child }, plus VectorPlacement / RasterPlacement ext traits providing at(pos) and anchored(self_anchor).snap_to(target_point). The anchored form mirrors Vec2::anchored(...).snap_to(...) so the geometry vocabulary carries over to component placement.
• tellur-core::layer: VectorLayer.children / Layer.children now hold Vec<Placed<dyn _>>; add takes a single Placed<dyn _> argument.
• tellur-core::geometry: rename Vec2::anchor → anchored (past-participle reads better at call sites). Drop AnchoredSize::snap_to_anchor — callers use snap_to(target_anchor.point(target_size)).
• Examples: all three reshaped to construct the layer as a single struct-literal with inline placement; timeline_to_mp4 also unrolls the fps loop into four direct entries. The (view_box().anchor().snap_to_anchor(...), boxed()) tuple form is gone.