docs: opaque white background and larger annotations in pipeline.svg

Address review feedback on the new landing-page illustration:

- Add an opaque white background rect covering the full viewBox so
  the entire illustration (annotations included) reads correctly
  against any page theme. Previously the code panel had its own white
  rect but the surrounding annotations sat on transparent — black
  text on a black page background made them invisible in GitHub's
  dark-mode README rendering.
- Bump the annotation font size from 17 to 19 to keep effective
  rendered text in the 9-11pt range when displayed at typical README
  column widths.

Re-render images/pipeline.png from the updated SVG.

Author: dimitri-yatsenko