Support nested SVG text spans

[rowland]

Problem

The current parser warns svg: <text>: nested text elements are unsupported and were skipped. This means SVGs using <tspan> or more structured text runs lose portions of their text.

What implementation would require

• Extend text parsing to preserve nested text nodes instead of flattening only direct character data.
• Add support for <tspan> as the first practical target, including inherited style, per-span x/y/dx/dy, and anchor behavior.
• Decide the minimum text layout subset we need now versus full SVG text semantics.
• Make renderer state changes safe across nested spans so fill/font changes do not leak.
• Add tests for multi-span text, partial style changes, and positioned tspans.

Workarounds

• Convert text to outlines for logo/branding assets.
• Export SVGs with simpler single-node text when text remains editable.
• Use plain <text> without nested spans in PDF-targeted assets.