feat: add Text component with per-span styling

[AsPulse]

Adds a Text vector component that shapes a sequence of styled spans through rustybuzz and emits one filled Path per glyph through the existing VectorGraphic pipeline. Per-span styling is expressed as overrides on each TextSpan (fill / font / size / weight), and the shaped spans can be pulled out as independently placeable graphics for per-span effects.

• tellur-core::text (new): Text { font, size, weight, fill, spans: Vec<TextSpan> }, with TextSpan holding text plus Option<Paint> / Option<Arc<Font>> / Option<f32> / Option<Weight> overrides. Spans inherit any unset field from the enclosing Text. Shaping runs through rustybuzz::shape; glyph outlines come from ttf_parser::Face::outline_glyph and are converted to PathCommands. Line metrics (ascender/descender/line_gap) come from the base font.
• Font: owns the font bytes via Arc<Vec<u8>> plus a face_index for .ttc collections. Constructors:
  • Font::from_bytes(bytes) / Font::from_bytes_indexed(bytes, index)
  • Font::load_file(path)
  • Font::find_by_name(name) / Font::find_by_name_with_weight(name, weight) via fontdb
  • Font::sans_serif() / serif() / monospace() / cursive() / fantasy() via fontconfig (the same machinery fc-match uses, so per-language / per-script overrides on the host are honoured rather than relying on a hardcoded fallback list).
• Static SANS_SERIF / SERIF / MONOSPACE / CURSIVE / FANTASY as LazyLock<Arc<Font>>: the OS resolution runs once on first access; subsequent uses are just Arc::clone.
• Weight newtype (CSS-style 100..900 with THIN / ... / BLACK constants) drives the OpenType wght axis via rustybuzz::Face::set_variations. Has visible effect on fonts whose wght axis renders correctly through ttf-parser; on others (or non-VF fonts), use find_by_name_with_weight to load the per-weight file from the family instead.
• Text::into_spans() -> Vec<Placed<TextSpanGraphic>> decomposes a shaped line into one independently placed component per input TextSpan. Each TextSpanGraphic is a VectorComponent whose layout box matches the span's own width × the line height, so callers can pattern-match let [a, b, c] = text.into_spans().try_into().unwrap(); and apply per-span effects (rasterize → outline / drop shadow / transform) before recomposing into a layer.
• tellur-core::layer: VectorLayer.size and Layer.size change from Vec2 to Option<Vec2>. None auto-fits to the children's combined paint bounds — view_box.origin (vector) / paint_rect.origin (raster) tracks the bounding rect's origin so children at negative positions are kept inside the box rather than clipped. ::fit() constructors mirror ::new(size). Auto-fit children are laid out under Constraints::UNBOUNDED; Fill-mode children there collapse to zero on each axis, which is the expected interaction.
• tellur-core::placement: impl From<Placed<C>> for Placed<dyn VectorComponent> (and the raster counterpart) for C: VectorComponent + 'static, so concrete-typed Placed<TextSpanGraphic> etc. flow into Vec<Placed<dyn _>> via .into() without a hand-written cast. This is what lets Text::into_spans()'s output drop straight into a VectorLayer.
• Dependencies: rustybuzz (shaping, re-exports ttf-parser), fontdb (system font database for find_by_name), fontconfig (FFI binding for find_generic — the dev shell's flake.nix gets pkgs.fontconfig so the linker resolves libfontconfig).
• Example: tellur-renderer/examples/text_to_png.rs (new) renders "Hello world!" with the middle span in red, centered on a 1920×1080 white canvas, font resolved via the SANS_SERIF static.