Playground v2: multi-file CodeMirror editor, fls diagnostics, real bug fixes

[omdxp]

Summary

• Fixed a real compiler regression: async main's shadow-renamed C entry point got a bogus conflicting prototype, breaking every async fun main() program (native, not just the website).
• Fixed the runnable code block's cursor-desync bug (manual resize let the overlay and textarea drift apart) and replaced the "Runnable Example" title bars with a floating hover toolbar, a line-number gutter, and auto-growing height.
• Fixed edited-code compile errors silently falling through to a generic "no backend" message instead of showing the real C compiler output.
• The Playground's wasm modules now load eagerly on page idle instead of the first click, with content-hash cache-busting per release.
• Consolidated the Playground from a flat list of per-sample run blocks into one multi-file project: a real file tree (add/rename-entry/delete), CodeMirror 6 editing with Fun syntax highlighting, and a compile pipeline that mounts every project file so cross-file imports resolve.
• The Playground editor now uses this project's own Fun Web/Fun Web Light color theme instead of CodeMirror's generic default.
• Wired real fls language server features into the editor, all through a generalized, protocol-agnostic wasm entry point (wasm_fls_main.fn replays any sequence of raw LSP messages, reload-per-call - see docs/platforms.md for why not a persistent server):
  • Diagnostics, debounced, theme-aware, falls back silently on any failure.
  • Hover - a symbol's signature on mouseover.
  • Completion - real suggestions from fls's own analysis of the open document.
  • Signature help - a call's parameter list while typing inside its parens.
  • Goto-definition - Cmd/Ctrl+click (or F12/Alt-D), including across project files.
• Audited the docs' runnable example corpus for staleness; found none beyond intentional non-standalone fragments and error-demonstration snippets.

Test plan

• fun test . - 56/56 test files pass (native rebuild with the async-main fix and the generalized fls wasm entry point)
• scripts/run_examples.sh - no new failures (2 pre-existing environmental flakes: local OpenSSL arch mismatch, documented BitInt flake)
• vscode extension smoke test - 9/9 pass (1 skipped, no VS Code host available)
• Every examples/advanced/async_*/channel_async_composed.fn example compiles and runs correctly with the async-main fix
• Real production build (vite build) served under /fun/ with real COI headers - multi-file sample run, new-file creation, and live diagnostics all verified end to end, zero console errors, zero failed requests
• Both light and dark themes verified for the new editor, lint gutter, diagnostic underlines, and the Fun Web/Fun Web Light syntax colors
• Hover, completion, signature help, and both same-file and cross-file goto-definition each verified with real wasm fls round trips in a real browser (not assumed from the response shape)