Accessibility 100 across both themes and both viewports

Two contrast failures, both from measuring a colour against the page ground
rather than the surface it actually sits on.

The dark --accent-ink (#cc99ff) cleared 4.5:1 on #333333 but only reached
3.94:1 on the accent wash that a pressed or hovered control sits on — which
is where the theme toggle lives. Lifted to #d9b3ff within the same hue:
7.1:1 on the page, 4.9:1 on the wash.

Adds tests/keyboard.e2e.mjs: a scripted keyboard-only walkthrough that
passes 10 checks with no pointer. Skip link to main; theme toggle as a real
button flipping aria-pressed; Ctrl+K opening search with focus moving in and
returning to the opener on Escape; search navigating to a provision; the
mobile contents sheet opening, trapping focus through a full 25-tab lap and
returning it on Escape; copy-link naming and its polite live region; one
main and one h1 per page type; a cold load of #art-11-s-2 landing clear of
the sticky header; a legacy #article5 link rewriting itself to #art-5; and
zero console errors across six page types.

puppeteer-core is pinned to 25.7.0 — 23.x pulls a vulnerable extract-zip
through @puppeteer/browsers (3 high advisories) for a browser downloader
this never uses. 25.7.0 reports 0 vulnerabilities and, despite declaring
Node >=22.12, imports and drives Chrome correctly on Node 18, verified
rather than assumed. e2e is a separate script, not part of npm test, and
skips cleanly where no Chrome exists.

Lighthouse accessibility is now 100 in all four combinations of
desktop/mobile and light/dark.

Author: KiranPranay