docs(ext-api): clarify pauseTracking/resetTracking no-op stub comment (review #12142.A5)

The previous comment described what the Vue intrinsics do but called them
no-op shims without explaining the contract gap. That read as if the
service was relying on dep-tracking suppression that the shim was silently
violating.

Rewrite the comment to make the Phase A scope explicit: this is a no-op
stub by design (the API surface is the contract being stabilized, not the
dep-tracking guarantee), and the real @vue/reactivity import lands together
with the reactive-World contract in #11939. Adds the TODO(#11939) marker so
the swap-in is grep-able alongside the rest of the Phase B handoff points.

No code change - comment-only. Per Architect review #5, we deliberately do
NOT add @vue/reactivity as a direct dep in Phase A.

src/services/extension-api-service.ts

@@ -18,9 +18,21 @@
 
 import { EffectScope, onScopeDispose, proxyRefs, watch } from 'vue'
 
-// pauseTracking/resetTracking prevent accidental reactive deps during setup hooks.
-// They are Vue internals not publicly exported. Use no-op shims so this service
-// compiles and runs without a direct @vue/reactivity dep.
+// Phase A no-op stub for the Vue `pauseTracking`/`resetTracking` pair.
+//
+// In Vue's `setupStatefulComponent` (component.ts:829-927) these calls bracket
+// `setup()` so that any property reads inside the user's setup body do NOT
+// register as reactive dependencies of the surrounding effect. We mirror that
+// pattern in the extension scope harness.
+//
+// We intentionally do NOT take a direct dependency on `@vue/reactivity` for
+// Phase A — the contract being stabilized here is the API surface, not the
+// dep-tracking guarantee. Real bracketing lands together with the reactive
+// World contract in #11939 (Phase B), at which point these stubs are replaced
+// by `import { pauseTracking, resetTracking } from '@vue/reactivity'`.
+//
+// TODO(#11939): swap in the real pauseTracking/resetTracking from
+// @vue/reactivity once the reactive-World contract is live.
 const pauseTracking = (): void => {}
 const resetTracking = (): void => {}