ie-shell: cache parsed page — don't re-parse HTML/CSS/layout on every redraw

[thomasnemer]

Goal

Currently render_page() runs the full pipeline (parse → style → layout → display list) on every RedrawRequested. This is extremely wasteful.

Current state

• Every frame: re-tokenizes HTML, re-builds DOM, re-resolves styles, re-layouts, re-builds display list
• On a complex page this takes 10-100ms per frame
• Window resize, overlay toggle, any redraw triggers full re-parse

Implementation needed

• Cache ParseResult + styles + layout tree on the active Tab
• Only re-parse when page source changes (navigation)
• Only re-layout on resize (viewport changed)
• Only re-build display list on layout change
• Overlay rendering doesn't need page re-parse

Impact

Major performance improvement. Currently browsing feels sluggish.