Use native fetch() polyfill from JsRuntimeHost in Playground (#1707)

Wires Babylon Native's Playground up to the **native `fetch()`
polyfill** that now lives in JsRuntimeHost
(BabylonJS/JsRuntimeHost#188), so playgrounds depending on `fetch` stop
hitting `ReferenceError: 'fetch' is not defined`.

This supersedes the original JS XMLHttpRequest-based shim approach:
rather than shipping a `fetch_polyfill.js` script, `fetch` is now
provided natively (built on the same `UrlLib` transport as
`XMLHttpRequest`), matching how File/FileReader landed in #1706.

**Changes:**
- `Apps/Playground/Shared/AppContext.cpp` - calls
`Babylon::Polyfills::Fetch::Initialize(env)` alongside the other
polyfills (right after `XMLHttpRequest`).
- `Apps/Playground/CMakeLists.txt` +
`Apps/Playground/Android/BabylonNative/CMakeLists.txt` - link the
`Fetch` target.
- `CMakeLists.txt` - bumps the JsRuntimeHost pin to pick up the native
polyfill.

> ✅ **Unblocked:** JsRuntimeHost#188 merged, and the pin now points at
`BabylonJS/JsRuntimeHost@a407a331` (the #188 merge commit) — no longer
the temporary fork branch. Rebased onto master (clean).

**Verified (Win32/Chakra):** `Playground` configures, builds and links
cleanly against the native polyfill, and at runtime `typeof fetch ===
"function"` (confirmed via a headless smoke check).

---

## Landing context

This PR is one of the **splits** from the proven CI-green combined
preview in draft PR #1702 (see
[#1702](#1702) for the
full intended end-state).

> The original split also included #1709 (ES2020+ -> ES2019
syntax-repair polyfill for Chakra), closed in favour of investigating
`@babel/standalone` properly (#1711).

### Landing order / status

**Tier 1 - parallel-reviewable, no source conflicts:**
1. ~~#1703 - ExternalTexture C4702 build fix~~ ✅ merged
2. ~~#1704 - config.json `reason` rewrites (5 entries)~~ ✅ merged
3. ~~#1705 - config.json `reason` rewrites (17 entries)~~ ✅ merged

**Tier 2 - sequential, each touches `Apps/Playground/CMakeLists.txt` +
`AppContext.cpp`:**

4. ~~#1706 - File/FileReader polyfill~~ ✅ merged (native JsRuntimeHost
bump)
5. **#1707 - native fetch polyfill (this PR)** - ✅ unblocked; pin
repointed to `BabylonJS/JsRuntimeHost@a407a331`; ready for review
6. #1708 - link AbortController + TextEncoder polyfills from
JsRuntimeHost
7. #1710 - Cubemap auto-expand polyfill (loaded after babylon.max.js)

### Reference policy reminder

Reference PNGs come from Babylon.js; never re-baked by BN. Combined
diff: **0 PNGs**.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bkaradzic-microsoft

Apps/Playground/Android/BabylonNative/CMakeLists.txt

@@ -30,6 +30,7 @@ target_link_libraries(BabylonNativeJNI
     PRIVATE Blob
     PRIVATE Canvas
     PRIVATE Console
+    PRIVATE Fetch
     PRIVATE File
     PRIVATE GraphicsDevice
     PRIVATE NativeCamera

Apps/Playground/CMakeLists.txt

@@ -156,6 +156,7 @@ target_link_libraries(Playground
     PRIVATE TextDecoder
     PRIVATE Window
     PRIVATE XMLHttpRequest
+    PRIVATE Fetch
     ${ADDITIONAL_LIBRARIES}
     ${BABYLON_NATIVE_PLAYGROUND_EXTENSION_LIBRARIES})
 

Apps/Playground/Shared/AppContext.cpp

@@ -20,6 +20,7 @@
 #include <Babylon/Polyfills/Blob.h>
 #include <Babylon/Polyfills/Canvas.h>
 #include <Babylon/Polyfills/Console.h>
+#include <Babylon/Polyfills/Fetch.h>
 #include <Babylon/Polyfills/File.h>
 #include <Babylon/Polyfills/Performance.h>
 #include <Babylon/Polyfills/TextDecoder.h>
@@ -186,6 +187,7 @@ AppContext::AppContext(
         Babylon::Polyfills::TextDecoder::Initialize(env);
 
         Babylon::Polyfills::XMLHttpRequest::Initialize(env);
+        Babylon::Polyfills::Fetch::Initialize(env);
         m_canvas.emplace(Babylon::Polyfills::Canvas::Initialize(env));
 
         Babylon::Plugins::NativeTracing::Initialize(env);

CMakeLists.txt

@@ -54,7 +54,7 @@ FetchContent_Declare(ios-cmake
     EXCLUDE_FROM_ALL)
 FetchContent_Declare(JsRuntimeHost
     GIT_REPOSITORY https://github.com/BabylonJS/JsRuntimeHost.git
-    GIT_TAG c88625b6d61b55c4589f02408d03826e83199870)
+    GIT_TAG 99457c03625782c3eeac6609f632538c7c9445d0)
 FetchContent_Declare(metal-cpp
     GIT_REPOSITORY https://github.com/bkaradzic/metal-cpp.git
     GIT_TAG metal-cpp_26