Merge origin/v3: tooltips, cursor glint, lighter…

Details

Brings the branch up to date with v3. One conflict, in _layouts/v3.html:
v3 added data-tip tooltips to the three floating display toggles, while this
branch had moved those buttons into the fixed footer dock. Resolved by keeping
the dock (the buttons' hover/focus pill already reveals a text label — a
tooltip on the same trigger would double-label them); the tooltip system
itself lands intact for the nav controls, alongside the cursor glint,
lighter hero scrim, Dark Reader opt-out, and the branch-name guard hook.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016HyukoK8KXyZw4hY9Wm3zN

Files changed

Integration (merge) commit — no direct file changes.