perf(dev): 6.5x faster dev boot + fix Windows spawn ENOENT

[Zen0-99]

Summary

• Gate the React Compiler to production builds only. It was running its full babel pass over every .tsx module on every dev start, accounting for a large share of the ~45s first-load transform cost.
• Preload the editor body chunk in parallel with the page render, and skip the 9 sibling workspace-page prewarms in dev (they forced ~20s of Vite transforms and starved the editor of CPU).
• Add a Windows fallback to scripts/lib/freePort.ts (netstat/tasklist) so the dev port probe works when lsof/ps are not on PATH.
• Fix scripts/dev.ts child spawning on Windows: bun is a .cmd shim and vite is not on the child's PATH, so Bun.spawn(['bun', ...]) threw ENOENT. Children now run through process.execPath (the real bun.exe) - bun for its own entry, bun executing node_modules/vite/bin/vite.js for the dev server.

Measured result: dev boot 47.7s -> 7.4s (6.5x).

Why

bun run dev presented two waits: a ~20s waiting screen before the admin UI painted, then a second wait for the editor chunk ("Editor is still loading - Retry"). An instrumented boot corridor showed the dominant cost was Vite's per-module transform in dev - largely the React Compiler babel pass on every module every start - plus the editor chunk being fetched serially after first paint and the sibling workspace pages being prewarmed up front. Separately, the Windows spawn bug (a .cmd shim) blocked bun run dev entirely with ENOENT.

Test

• bun run dev boots clean: [server] Listening on http://localhost:3001 and VITE ready in ~4s - no ENOENT.
• Editor is interactive on /admin/site with no Retry screen; the sibling workspace pages (PluginPage, AiPage, ...) are gone from the first-load wait.
• Confirmed process.execPath resolves to the real bun.exe, node_modules/vite/bin/vite.js exists, and Bun.which('vite') is null (so the explicit path is required).
• bun test src/tests/architecture/bundle-size-budgets.test.ts unaffected: the editor body stays a separate lazy chunk (preload changes fetch timing, not bytes).