[JSC] Walk star-export graph once when building a module namespace
Source/JavaScriptCore/runtime/AbstractModuleRecord.h
ES module namespaces are constructed by GetModuleNamespace, which must enumerate all names reachable through transitive export * chains. The spec's ResolveExport is recursive and carries a resolveSet to break cycles. The old implementation called this per-name, redundantly re-walking the star-export graph for every name — O(names × star-edges). The new implementation fuses the graph walk with binding collection: it traverses each star edge once, accumulates per-name candidate sets, and fast-paths names with exactly one Local/Namespace binding as Resolved. Indirect entries fall back to resolveExport(); ambiguous names with two-or-more distinct bindings are excluded. Benchmarks show ~17x for deep barrel modules with 9000 exports across ~1500 star edges.
Significance
This restructures a core spec algorithm with a fast path whose correctness depends on non-obvious invariants about binding uniqueness and shadow rules — exactly the kind of change that can introduce subtle divergence from spec behavior.