fix(runtime): Promise(data) branch missing symbol_descriptors accessor check in get_computed_property_impl

[dowdiness]

Context

PR #413 fixed the Array(arr) branch in interpreter/runtime/property.mbt's get_computed_property_impl — it was checking symbol_properties directly without first consulting symbol_descriptors for getter-type (accessor) entries. The same gap exists in the Promise(data) branch and was not addressed in that PR.

Identified during the PR #413 post-merge analysis.

Problem

In interpreter/runtime/property.mbt, the per-variant dispatch in get_computed_property_impl follows this two-step protocol for own symbol property lookup:

1. Check symbol_descriptors for an accessor descriptor (getter). If a getter is present, call it.
2. Check symbol_properties for a data value.

The Object, Map, and Set branches all follow this protocol. The Array branch was fixed in PR #413. The Promise(data) branch (currently around line 1031) only performs step 2 — it reads data.bag.symbol_properties directly and skips data.bag.symbol_descriptors entirely:

Promise(data) => {
  match data.bag.symbol_properties.get(sym.id) {  // step 2 only — missing step 1
    Some(v) => return v
    None => ()
  }
  ...
}

This means if a Promise instance has an accessor-type own symbol property (e.g., a getter installed on its bag.symbol_descriptors), the getter will be silently skipped and the property will read as absent.

Current severity: Low — no code path in the engine currently installs getter-type symbol properties on individual Promise instances. However, this will become exploitable as Proxy wrapping and future spec features (Observable, async iterator protocol extensions) create Promise-like objects with intercepted symbol access.

Suggested work

Mirror the fix applied to Array(arr) in PR #413. Before the existing symbol_properties lookup in the Promise(data) branch, add a symbol_descriptors accessor check:

Promise(data) => {
  // Check for accessor descriptor (getter) on own symbol property.
  match data.bag.symbol_descriptors.get(sym.id) {
    Some(desc) =>
      match desc.getter {
        Some(getter) => return self.call_value(getter, obj, [], loc)
        None => ()
      }
    None => ()
  }
  // Check promise's own symbol property bag.
  match data.bag.symbol_properties.get(sym.id) {
    Some(v) => return v
    None => ()
  }
  ...
}

Also audit whether the has_property path for Promise(data) has the same gap (line ~665 area — the hasOwnProperty implementation), and whether set_computed_property_impl needs a matching fix for the setter side.

Acceptance criteria

• get_computed_property_impl Promise(data) branch checks symbol_descriptors before symbol_properties
• All existing Promise tests still pass (1272/1272)
• moon test clean
• A whitebox test in interpreter/runtime/ pins the behavior: a Promise instance with a getter-type symbol descriptor correctly fires the getter