Add editorial reflow demo with drop cap and pull quote

[yuribodo]

Summary

• New demo page at /demos/old-man-sea — an editorial reading experience that showcases layoutNextLine() for variable-width text flow
• Drop cap: large initial letter with body text flowing around it for the first three lines
• Pull quote: positioned as a mid-flow obstacle on the right side, body text wraps around it using per-line width adjustment
• Live reflow: resize the window and all text reflows in real time with zero DOM reads in the layout path
• Responsive: pull quote hides on narrow viewports (<420px stage width)
• Performance pill: shows reflow timing and line count live
• Added card to the demo index page

This demo dogfoods layoutNextLine() for all body text, exercising the streaming variable-width layout path as suggested in AGENTS.md. The "Old Man and the Sea" resize/reflow demo was listed as a priority in TODO.md.

Test plan

• bun run check passes (tsc + oxlint)
• bun test passes (60/60 unit tests)
• Open /demos/old-man-sea and verify drop cap, pull quote, and text rendering
• Resize the window horizontally — text should reflow smoothly
• Resize to narrow width (<400px) — pull quote should disappear gracefully
• Check performance pill shows sub-millisecond reflow times
• Verify text is selectable