feat: Lua app firmware for the E32R40T display module

Boots to a launcher that lists /apps/<name>/main.lua on the SD card and runs
the selected app in a vendored Lua 5.4 with gui, input, fs, sys and log
bindings. Settings persist as a Lua table in /settings.lua, covering touch
calibration and screen rotation, with a settings app to edit both. Rotation is
applied after mapping raw touch into the panel's rotation-0 frame, so turning
the UI never invalidates a calibration.

src/settings.cpp

@@ -0,0 +1,63 @@
+#include "settings.h"
+
+#include <SD.h>
+
+extern "C" {
+#include <lauxlib.h>
+#include <lua.h>
+}
+
+Settings settings;
+
+static constexpr const char* PATH = "/settings.lua";
+
+static int16_t fieldOr(lua_State* L, const char* key, int16_t fallback) {
+  lua_getfield(L, -1, key);
+  int16_t value = lua_isinteger(L, -1) ? lua_tointeger(L, -1) : fallback;
+  lua_pop(L, 1);
+  return value;
+}
+
+bool Settings::setRotation(int16_t degrees) {
+  if (degrees % 90 != 0 || degrees < 0 || degrees > 270) return false;
+  rotation = degrees;
+  return true;
+}
+
+bool Settings::load() {
+  File f = SD.open(PATH);
+  if (!f) return false;
+  String source = f.readString();
+  f.close();
+
+  lua_State* L = luaL_newstate();
+  if (!L) return false;
+  bool ok = luaL_loadbuffer(L, source.c_str(), source.length(), PATH) == LUA_OK &&
+            lua_pcall(L, 0, 1, 0) == LUA_OK && lua_istable(L, -1);
+  if (ok) {
+    rotation = fieldOr(L, "rotation", rotation);
+    lua_getfield(L, -1, "touch");
+    if (lua_istable(L, -1)) {
+      touchX0 = fieldOr(L, "x0", touchX0);
+      touchY0 = fieldOr(L, "y0", touchY0);
+      touchX1 = fieldOr(L, "x1", touchX1);
+      touchY1 = fieldOr(L, "y1", touchY1);
+    }
+    lua_pop(L, 1);
+  }
+  lua_close(L);
+  return ok;
+}
+
+bool Settings::save() const {
+  File f = SD.open(PATH, FILE_WRITE);
+  if (!f) return false;
+  char buf[200];
+  int len = snprintf(buf, sizeof(buf),
+                     "return {\n  rotation = %d,\n"
+                     "  touch = { x0 = %d, y0 = %d, x1 = %d, y1 = %d },\n}\n",
+                     rotation, touchX0, touchY0, touchX1, touchY1);
+  bool ok = f.write(reinterpret_cast<const uint8_t*>(buf), len) == (size_t)len;
+  f.close();
+  return ok;
+}