Add compat flag docs for FinalizationRegistry and WeakRef

src/content/changelog/workers/2025-04-24-finalization-registry.mdx

@@ -0,0 +1,11 @@
+---
+title: Improved Memory Efficiency for WebAssembly Workers
+description: With the introduction of FinalizationRegistry in Workers Runtime, toolchains like Emscripten and wasm-bindgen can automatically reclaim unused WebAssembly heap and reduce memory leaks.
+products:
+  - workers
+date: 2025-05-24T00:00:00Z
+---
+
+[FinalizationRegistry](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry) is now available in Workers (opt-in with the `enable_weak_ref` compat flag), so toolchains such as Emscripten and wasm-bindgen can automatically free WebAssembly heap allocations.
+
+While this will reduce long-running memory growth and leaks to a certain extent, the API still depends on garbage-collector timing and other non-deterministic factors, so treat it as a helpful mitigation rather than a guaranteed fix.

src/content/compatibility-flags/js-weak-refs.md

@@ -0,0 +1,24 @@
+---
+_build:
+  publishResources: false
+  render: never
+  list: never
+
+name: "Enable `FinalizationRegistry` and `WeakRef`"
+sort_date: "2025-05-05"
+enable_date: "2025-05-05"
+enable_flag: "enable_weak_ref"
+disable_flag: "disable_weak_ref"
+---
+
+Enables the use of [`FinalizationRegistry`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry) and [`WeakRef`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakRef) built-ins.
+* `FinalizationRegistry` allows you to register a cleanup callback that runs after an object has been garbage-collected.
+* `WeakRef` creates a weak reference to an object, allowing it to be garbage-collected if no other strong references exist.
+
+:::note[Behaviour]
+`FinalizationRegistry` cleanup callbacks may execute at any point during your request lifecycle, even after your invoked handler has completed (similar to `ctx.waitUntil()`). These callbacks do not have an associated async context — you cannot perform any I/O within them, including emitting events to a tail Worker.
+:::
+
+:::caution
+These APIs are fundamentally non-deterministic — the timing and execution of garbage collection are unpredictable, and you **should not rely on them for essential program logic**. Additionally, cleanup callbacks registered with `FinalizationRegistry` may **never be executed**, including but not limited to cases where garbage collection is not triggered, or your Worker gets evicted.
+:::