Add a generic GC module (gc.rs) with tri-color marking (white/gray/black), heap allocation via Vec with free-list reuse, and stop-the-world collection triggered when live object count exceeds a configurable threshold. Refactor VM to use GcRef handles for heap objects (plain objects and functions) instead of owned values. This gives correct JS reference semantics — object assignment shares identity rather than deep-cloning. The GC traces through object properties and prototype chains to find all reachable objects, and correctly collects cycles. Key changes: - gc.rs: Generic Gc<T: Traceable> with alloc/get/collect, HeapSlot with color metadata, mark phase with gray stack, sweep with free-list - vm.rs: Value::Object(GcRef) and Value::Function(GcRef) replace owned data; VM allocates through self.gc and triggers collection after allocations; root collection scans registers and globals - lib.rs: evaluate() uses to_js_string(&gc) for GC-aware string conversion 8 new GC tests (alloc, collect, transitive reachability, cycle collection, free-list reuse, threshold, stress) plus 3 integration tests. All 226 existing tests continue to pass. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>