Real source-phase imports

Date: 2026-06-10 Area: engine

Source-phase imports now use a real JavaScript module source provider instead of only recognising the test262 <module source> sentinel. LoadModuleSourceValue resolves through the normal module resolver, parses JavaScript module source for early errors without linking or evaluating it, caches a ModuleSource object by resolved source-phase request key, and reports non-JavaScript module kinds as having no source representation. language.md.