Anchor annotations by selector + source location, never coordinates
Decision. Annotations anchor to a CSS selector plus the
data-astro-source-file / data-astro-source-loc mapping Astro dev mode
already injects. Pixel coordinates are never stored.
Why. Pages reflow constantly during dreaming; pixel anchors drift into nonsense within an edit or two — the known failure mode of Vercel-style preview comments. Selector + source survives reflow and usually survives refactors.