feat(a11y): make the deck keyboard and screen-reader navigable

[Reebz]

What

The deck was not usable by keyboard or screen-reader users. Nav dots were non-focusable divs, inactive slides stayed in the accessibility tree, and slide changes were silent.

• Nav dots are now <button type="button"> with aria-label="Go to slide N of M" and aria-current on the active dot, so they take focus and activate on Enter and Space for free.
• Inactive slides carry inert and aria-hidden, so a screen reader reads only the active slide and Tab skips hidden controls.
• Focus moves to the active slide on navigation, and each change is announced through a visually hidden aria-live="polite" region.
• CSS adds a button reset so the dots look unchanged, a :focus-visible ring visible only for keyboard users, and an .sr-only utility for the live region.

Mirrored into assets/slides-runtime.js and the inline runtime in references/html-template.md, including the .sr-only and focus styles, with the contract documented in references/presentation-layer.md.

Test

• Dots are reachable by Tab and activate on Enter and Space, moving to the right slide.
• Only the active slide is in the accessibility tree, and Tab does not reach controls inside hidden slides.
• Changing slides announces "Slide N of M" through the live region.
• The focus ring shows for keyboard users and not for mouse users.

Dependencies

Touches slides-runtime.js, so it is cleanest to merge after the nav-input-hardening PR.

Co-Authored-By: Leslie Barbara Knope (Claude Opus 4.8 (1M context)) noreply@anthropic.com

[vercel]

@Reebz is attempting to deploy a commit to the bluedusk's projects Team on Vercel.

A member of the Team first needs to authorize it.