Scene.set() JS lifecycle, UI sprite caching, emulator test scripts

- Add Scene.set(module) so JerryScript can switch scenes via
  require()'d {init, update, dispose} modules; wire it into
  engineUpdate() and rework overworldscene.js/init.js to the new shape.
- Fix scriptManagerExecFile() never pushing its own file's directory
  onto the require() dir stack, breaking relative require() calls from
  a top-level entry script (e.g. init.js -> ./overworldscene.js).
- Cache sprite geometry across frames for uislider/uitab/uiframe
  (dialogs/textbox/settings) and give uitextbox a per-page glyph cache
  instead of one spriteBatchBuffer call per visible character. uiconsole
  drops its alloc/free vertex buffer for a fixed-size one. uifps skips
  rebuilding its label when the text hasn't changed.
- Add PSP_OPTIMIZATION_PLAN.md and a handful of UI/spritebatch unit
  tests covering the new caching logic.
- Add Dolphin/PPSSPP emulator smoke-test scripts (+ Docker variants and
  CI jobs) for GameCube/Wii/PSP.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/scripts/init.js

@@ -0,0 +1,9 @@
+// Copyright (c) 2026 Dominic Masters
+//
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+// Entry point run once at startup (see game.c). Swap which scene module
+// gets passed to Scene.set() here to change what the game boots into.
+var overworldScene = require('./overworldscene.js');
+Scene.set(overworldScene);