Preserve SVG cursor colors and captured line spacing

[jackspirou]

SVG output now preserves the selected theme cursor color and captured line spacing. A custom cursor color previously used the text color. LineHeight 1.5 previously multiplied the measured row height again, shifting text and colored backgrounds beyond their captured rows.

The repair uses the authored cursor color with existing text/default fallbacks. It applies line height once when it derives fallback metrics and uses captured metrics directly. It also escapes the cursor fill attribute and corrects the output-optimization example.

This restores style benefits from the original SVG proposal #1. Core SVG output, CSS animation, frame reuse, examples, and generation tests remain present.

Validation:

• Real terminal capture reproduces both defects before the repair. Chrome pixels verify cursor colors and fallbacks. Captured row tests verify text, colored backgrounds, and viewport dimensions.
• Full native race suite passes 303 cases and 99 top-level tests, with no failed or skipped tests. Root verification passes 24 focused cases.
• Vet, lint, formatting, and diff checks pass. The changed README comment adds no prose findings to the existing baseline. Pre-PR autoreview passes with no P0 findings. The exact-content secret scan is clean.