Figure-to-SVG renderer.

to_svg/1 takes an %Bland.Figure{} and returns the finished SVG as a binary. All geometry is computed here; series only carry data, and the renderer owns projection, tick placement, pattern defs, and the layout of ornaments.

Broad layout, outside-in:

┌── page border ────────────────────────────────────────┐
│  TITLE                                                │
│  subtitle                                             │
│         ┌── plot frame ─────────────────┐             │
│         │                               │   [legend]  │
│   y     │                               │             │
│   lbl   │          plotting area        │             │
│         │                               │             │
│         └───────────────────────────────┘             │
│                   x label                             │
│                                        [title block]  │
└───────────────────────────────────────────────────────┘