added server-side canvas support & Created Masonry Demo

[Unmesh100]

Masonry Demo :

• pages/demos/masonry.ts — Logic that computes card heights with Pretext.layout() without any DOM reads, then routes cards to the shortest column
• pages/demos/masonry.html — Dark-themed aesthetic with image-color cards, fixed bottom controls for column adjustment
• Updated pages/demos/index.html — Enabled the previously-linked but missing demo
  Why this matters:
• Masonry layouts typically require expensive DOM measurement (getBoundingClientRect) to know heights before placement
• This demo routes cards to the shortest column using heights computed purely from canvas measurement — zero DOM reads in the hot path
• The column slider demonstrates dynamic reflow without any reflow penalty
  Tech highlights:
• Uses layout(prepared, maxWidth, lineHeight) for height prediction
• Cards have variable heights based on their text content
• Column assignment via greedy shortest-column algorithm
• Responsive column count auto-adjusts to viewport