Add mquickjs variant support with host function callbacks

[justjake]

Summary

• Add mquickjs (MicroQuickJS) as a new variant option for quickjs-emscripten
• Implement host function callbacks (vm.newFunction()) for mquickjs
• Add runtime feature detection API (hasSupport(), assertHasSupport())

Details

mquickjs Variant

mquickjs is a minimal QuickJS fork targeted at embedded systems (~10KB RAM, ~100KB ROM). It lacks:

• Modules (ES6 import/export)
• Promises
• Symbols (built-in)
• BigInt
• Intrinsics configuration

Host Function Callbacks

mquickjs requires compile-time stdlib customization since its function table is ROM-resident. This PR adds:

• c/qts_mquickjs_stdlib.c: Custom stdlib generator that includes qts_host_trampoline in the function table
• Build rules in Variant.mk to compile the stdlib generator with host cc and generate the header with -m32 for wasm32
• QTS_NewFunction implementation using JS_NewCFunctionParams to create closures with host_ref_id

Feature Detection

Runtime API to detect which features are available in each variant:

if (QuickJS.hasSupport("modules")) {
  // Use module features
}

// Or throw if feature not available
QuickJS.assertHasSupport("promises", "evalCode with module flag");

Test plan

• Build mquickjs-wasmfile-debug-sync variant successfully
• Basic eval works: vm.evalCode('1 + 2') returns 3
• Host functions work: vm.newFunction('add', ...) can be called from JS
• Regular quickjs variants still build and work correctly

🤖 Generated with Claude Code