Add a README hero banner

[jdatcmd]

Adds a hero banner to the top of the README.

assets/hero.svg — a self-contained SVG (1200×470):

• The thesis is the swap. The centerpiece is a real git diff, - … LANGUAGE plpgsql / + … LANGUAGE uplpgsql, so the drop-in migration reads at a glance.
• The claim, backed by evidence. The 9–23× headline sits beside all nine real benchmark ratios from bench/comparison.md (23.0× Mandelbrot down to the honest 1.4× numeric row).
• Two accents carry the story. Cool ground = interpreted; warm amber = compiled/native. The upl prefix is amber over a neutral pgsql.
• Renders everywhere. No webfonts, no scripts — monospace + system-sans only, on a fixed dark ground that reads well in both GitHub themes. Sized close to the social-preview ratio, so it can double as the repo's social card.

Only two files change: the new assets/hero.svg and a centered <img> at the top of README.md. No code or behavior touched.

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