Playground: compare selected systems

[alexey-milovidov]

Summary

Adds a compare selected systems feature to the playground, plus a set of usability fixes discovered while building it.

Compare selected systems

• Shift / Ctrl / Cmd-click, right-click, or long-press (touch) a system to toggle it into a multi-selection, highlighted bright blue. A plain click/tap clears the selection.
• When a selection exists, Run all arms yellow as "Run selected (N)" and the competition runs only among the selected systems.
• The selection is remembered in the URL (?compare=a,b,c), so a shared link reopens with the same systems highlighted and the button armed.
• A discoverability hint explains the gestures — a top-right pill on hover (pointer devices) and a static caption on touch devices.

Other improvements

• Remember the selected example query in the URL (?example=N), restored on reload / shared links.
• Scroll to results on Run / Run all — smooth-scrolls to the output or leaderboard (mobile usability).
• Competition results panel fits its content, growing up to the viewport height then scrolling inside the rail (no more forced full-height / page-level vertical scroll).
• Fix a persistent horizontal scrollbar: the per-slab uptime balloons were laid out (visibility:hidden) at all times, inflating the page scroll width; they're now rendered only on hover (display:none). Also stopped flex from shrinking slabs and capped the example <select> width.
• file:// fallback: when opened directly from disk, fall back to the production API if no local dev server is running.

All changes are confined to playground/ (app.js, index.html, style.css), with cache-busting versions bumped.

Test plan

• Multi-select via all four gestures; verify bright-blue highlight, armed "Run selected (N)", and competition running only the selection.
• Reload with ?compare=…&example=… and confirm the systems + query are restored.
• Verify no horizontal/vertical scrollbars across widths (mobile → desktop) and that the competition rail fits content / scrolls internally when long.
• Run / Run all and confirm the page scrolls to the results.

🤖 Generated with Claude Code