fix(lua): stream scripts off the SD card instead of buffering them whole

Reading a module into one String needs that many bytes contiguous. Once WiFi is up the
largest free block drops to ~40KB, so loading /lib/ui.lua failed -- and readString()
reports allocation failure by silently returning a partial read, which reached Lua as a
syntax error at a random offset. Every app launch was broken while connected.

loadScript() now feeds lua_load() through a 512 byte buffer, so no module needs a
contiguous allocation. readScript(), still used by fs.readFile, verifies the length it
got against the file size rather than trusting it. The launch log carries free and
largest-block so the next heap question is answerable without a rebuild.

src/main.cpp

@@ -50,7 +50,7 @@ void startApp(const String& path) {
   // Full panel until the app's own status bar module reports its height in load().
   tft.setRotation(settings.rotationIndex());  // apps may have rotated the frame
   tft.resetViewport();  // load() re-clips once the new app's bar reports its height
-  Serial.printf("launching %s\n", path.c_str());
+  Serial.printf("launching %s free=%u largest=%u\n", path.c_str(), ESP.getFreeHeap(), ESP.getMaxAllocHeap());
   if (app.load(path.c_str(), path == HOME)) return;
   if (path == HOME) fallbackScreen("home failed to start");
 }