JS Promise and microtask queue #273

Implements issue 3mhn3oxz6dq2k — JS Promise and microtask queue.

Summary #

• Promise constructor: Defined via JS preamble so the executor callback runs as bytecode (not limited to native-only callbacks). Executor runs synchronously, with resolve/reject closures that call native __Promise_resolve/__Promise_reject helpers.
• Promise.prototype.then/catch/finally: Register reaction handlers on pending promises or enqueue microtasks immediately for already-settled promises. Returns chained promises for proper chain propagation.
• Promise.resolve/reject: Static methods to create pre-settled promises.
• Promise.all/allSettled/race/any: Combinators for working with arrays of promises.
• Microtask queue: Thread-local queue (Vec<Microtask>) drained after each vm.execute() call. Each microtask calls a handler function and resolves/rejects the chained promise with the result.
• VM.call_function: New method to invoke GC-managed functions (both native and bytecode) from outside the normal execution loop. Saves/restores frame state for bytecode calls.
• Promise.prototype as GC root: Stored on the Vm struct (like other prototypes) to prevent garbage collection.
• Removed Promise from test262 unsupported features list.

Test plan #

• 29 new integration tests covering:
  • Promise.resolve/reject with then/catch handlers
  • Constructor with executor (resolve, reject, synchronous execution)
  • Then chaining (value propagation through chain)
  • Catch returns to then chain (error recovery)
  • Error propagation (throw in then handler goes to catch)
  • Finally handler (fulfilled and rejected paths)
  • Promise.all (empty, resolved, rejects on first failure, non-promise values)
  • Promise.race (first wins, reject wins, non-promise values)
  • Promise.any (first fulfilled, all rejected)
  • Identity passthrough (then with no handler)
  • Catch passthrough (doesn't intercept fulfillment)
  • Double resolve ignored
  • Executor throw rejects
  • Multiple then on same promise
• All workspace tests pass (471 JS tests including 29 new Promise tests)
• cargo clippy --workspace -- -D warnings clean
• cargo fmt --all --check clean