The old mtime-tracking staleness machinery (_tui_build_needed,
_hermes_ink_bundle_stale, _find_bundled_tui) tried to avoid rebuilding
by comparing source timestamps to dist/entry.js. This was fragile and
added ~100 lines of code. Replace with three clear paths:
1. HERMES_TUI_DIR set (prebuilt/nix): just node dist/entry.js, no build
2. --dev mode: tsx src/entry.tsx, no build, hot reload
3. Normal: always npm run build (esbuild is ~1s, correctness > caching)
Also error when HERMES_TUI_DIR is set with --dev (footgun: prebuilt
bundle has no source code to hot-reload).
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