docs(ENG-278): Preserve scroll position and fix algo-ref anchor navigation

[alnoki]

Changes

1. Add scrollPreserve.js theme module that saves scroll offset
   to sessionStorage on .tex/.md/test-case changes, then
   restores it after reload by polling with requestAnimationFrame
   until async algorithm content is tall enough to scroll to
2. Replace the dev server full-reload WebSocket event with a
   custom algo-reload HMR event, extending scroll-preserving
   reloads to all index rebuild triggers (.tex, .md, .rs)
3. Add handleAlgoRefClicks in Algorithm.vue to intercept
   same-page #algo-ref- links and scroll directly to the target,
   bypassing VitePress router which misses dynamically rendered
   anchors
4. Re-scroll to #algo-ref- hash targets after each Algorithm
   finishes rendering, progressively correcting the offset as
   async content fills in (suppressed during scroll restoration
   via isRestoring())
5. Add scroll-margin-top CSS for [id^="algo-ref-"] elements
   so the fixed navbar does not cover anchor targets
6. Document scroll preservation in docs-engine.md with a new
   section and an <Include> of the source module, and update
   the Algorithm index builder section to reference the new
   reload behavior

[linear]

ENG-278