feat: add component body abstraction with proc macros

[AsPulse]

React/SwiftUI-style component composition. User-defined composites can now return another component instead of constructing a VectorGraphic themselves, and #[vector_component] / #[raster_component] proc macros collapse the struct + impl boilerplate into a single function.

• tellur-core::vector::VectorBody and tellur-core::raster::RasterBody: new enums distinguishing element components (own a render() implementation — Rectangle, Circle, Ellipse, VectorLayer, Layer, Rasterize<V>) from composite components that delegate via Body::Of(Box<dyn ...>).
• VectorComponent / RasterComponent traits gain default body() returning Body::Element and default render() that walks the body chain. target: Resolution threads through RasterBody::Of automatically — composites never see it in their signature.
• tellur-macros: new proc-macro crate exposing #[vector_component] and #[raster_component]. Each turns a function-style component definition into a PascalCase struct (fields = function args, #[derive(Clone)]) plus the trait impl. Re-exported from tellur-core.
• Examples migrated: BouncingDot (in timeline_to_mp4) and Blob (in raster_layer_to_png) are now written as plain functions; BouncingDot drops ~16 lines of boilerplate, Blob shrinks to 7 lines from 20.
• CI: tellur-macros added to the per-crate fmt / clippy / build / test matrix.