Send the merge confetti in from the page edges

The celebration on a landed change opened with a burst from the middle of the page, on top of the title and the text the reader had just arrived to read. This keeps the confetti at the edges of the window and nowhere else.

• Every particle now starts at the left or right edge and travels inward, so the celebration frames the page instead of covering it.
• The side volley carries the moment on its own: it starts at 60 particles per side instead of 30, so the total is still around five hundred.
• Nothing changes about how the celebration is loaded or drawn: still the same library, still fetched only for a page that carries ?merged=1, still one second, still honours reduced motion, still drawn on the main thread so the CSP does not need a worker exception.
• docs/ui-plan.md describes what the page now does.

Checks: vole workflow check and vole workflow run verify pass on this branch.