Part of #617. Blocks EngineSquared/xrepo#1.

Problem

The learning track consumes the engine as the enginesquared xrepo package, so a learner's first xmake run is that package's build. EngineSquared/xrepo#1 asks for a CoreOnly config on the package, but a package config only becomes an xmake f --CoreOnly=y flag on the engine build (package/tools/xmake.lua, install()), and this repository has no such option.

The root xmake.lua declares every dependency unconditionally (xmake.lua:12-30: joltphysics, wgpu-native, rmlui, freetype, zlib, glfw, glfw3webgpu, stb, miniaudio, lodepng, tinyobjloader) and includes every plugin (xmake.lua:34), so asking for EngineSquaredCore alone still pays for the whole graphics, physics, audio and UI stack. That first build is where a beginner quits.

What to build

A CoreOnly option in the root xmake.lua:

xmake f -y --CoreOnly=y

When it is on:

• only the dependencies the core and the utils need are required — entt, gtest, spdlog, glm, fmt
• src/engine and src/utils are included, src/plugin/* is not
• the EngineSquared aggregate target is skipped, since it depends on every plugin

When it is off, nothing changes.

Done when

• xmake f -y --CoreOnly=y resolves the five light packages only, and no plugin target is configured
• xmake f -y --CoreOnly=n resolves the full dependency set and the full target list, unchanged from today
• xmake build EngineSquaredCore succeeds in both configurations
• the core unit tests still build and pass with the option on
• verified on Linux, macOS and Windows