- Add proper try/catch compilation (PushExceptionHandler/PopExceptionHandler
bytecode ops) — previously try/catch was a stub that compiled blocks
sequentially without exception handling
- Add function property support (functions are objects in JS and can have
arbitrary properties like assert.sameValue)
- Implement Test262 harness preamble with assert, assert.sameValue,
assert.notSameValue, assert.throws, and Test262Error helpers
- Add evaluate_with_preamble API for running tests with harness globals
- Add instruction limit to VM to prevent infinite loops in test suite
- Implement test metadata parsing (features, flags, includes, negative tests)
- Add feature-based test skipping for unimplemented features
- Report pass/fail/skip grouped by category with pass rate percentages

Initial pass rate: 3406/8929 executed (38%), 14455 skipped.
Notable: keywords 100%, punctuators 100%, ASI 95%, block-scope 88%.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add property descriptors with writable/enumerable/configurable flags,
replacing the plain HashMap<String, Value> with HashMap<String, Property>.
This gives correct JavaScript semantics for property access, deletion,
and enumeration.

Key changes:
- Property struct with data descriptor flags (writable, enumerable, configurable)
- ObjectData gains extensible flag for future Object.preventExtensions
- FunctionData gains prototype_obj for instanceof support
- instanceof walks the prototype chain checking constructor.prototype
- delete respects the configurable flag (non-configurable = returns false)
- Compiler properly emits Delete bytecode for member expressions
- for-in loops fully implemented with ForInInit/ForInNext bytecode ops
- Property enumeration: integer indices first (sorted), then string keys
- Non-enumerable properties (e.g. array length) excluded from for-in
- SetPrototype/GetPrototype bytecode ops for prototype manipulation
- CreateClosure creates a .prototype object with constructor back-reference

15 new tests covering property descriptors, prototype chain, typeof,
instanceof, in, delete, for-in, enumeration order, and reference semantics.
All 241 JS tests pass (was 226).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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>