Add fetch() polyfill over XMLHttpRequest for Playground

[bkaradzic-microsoft]

Adds a JS polyfill for fetch so playgrounds depending on it stop hitting ReferenceError: 'fetch' is not defined.

Changes:

• New Apps/Playground/Scripts/fetch_polyfill.js - full fetch(input, init) wrapping XMLHttpRequest with Response-like text()/arrayBuffer()/json()/blob(). Self-detecting (no-op if global fetch already exists). Tailored to BN's XHR which dispatches only via addEventListener and fires loadend (no load). Decodes ArrayBuffer for .text() when responseType is arraybuffer.
• Apps/Playground/CMakeLists.txt - adds fetch_polyfill.js to SCRIPTS list.
• Apps/Playground/Shared/AppContext.cpp - loads fetch_polyfill.js before file_polyfill.js.

Note: Polyfill resolves the headline error but re-enables 0 tests. 5 affected playground tests still excluded - they hit secondary blockers (4 MSDF tests need babylonjs-addons, idx 151 needs earcut). Tracked as follow-ups.

Verified: regression sweep clean on idx 0/5/10/47/100/165/200.

Landing context

This PR is one of 7 splits from the proven CI-green combined preview in draft PR #1702 (see #1702 for the full intended end-state and verified CI run 26044922430).

Note: the original split included an 8th PR (#1709, ES2020+ -> ES2019 syntax-repair polyfill for Chakra). It was closed in favour of investigating @babel/standalone properly (#1711).

Recommended landing order

Tier 1 - parallel-reviewable, no source conflicts:

1. Fix ExternalTexture_OpenGL throw-stubs to avoid MSVC C4702 under /WX #1703 - ExternalTexture C4702 build fix
2. Document accurate root cause for post-#1695 pixel-diff fallouts #1704 - config.json reason rewrites (5 entries)
3. Document accurate root cause for 17 subtle pixel-diff tests #1705 - config.json reason rewrites (17 entries)

Tier 2 - sequential, each touches Apps/Playground/CMakeLists.txt SCRIPTS list + Apps/Playground/Shared/AppContext.cpp LoadScript order; rebase the next branch after the previous merges:

4. Add File/Blob/FileReader polyfill for Playground (re-enables 19 GLTF tests) #1706 - File/Blob/FileReader polyfill (largest test impact: 19 re-enables)
5. Add fetch() polyfill over XMLHttpRequest for Playground #1707 - fetch polyfill
6. Add DOM globals polyfill + native AbortController for Playground #1708 - DOM globals + native AbortController + Android CMake link
7. Add cubemap auto-expand polyfill for Playground (re-enables 7 PBR tests) #1710 - Cubemap auto-expand polyfill (loaded after babylon.max.js)

Reference policy reminder

Reference PNGs across all 7 PRs come from Babylon.js; never re-baked by BN. Combined diff: 0 PNGs.

[ryantrem]

Similar question as the other PR - should we just add a full native fetch polyfill? Maybe we could add it to XMLHttpRequest (@bghgary might say no though 😅), or a new polyfill. It would be good to have this in BN anyway so the BJS codebase can use fetch rather than XMLHttpRequest.

[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

Same as #1700 and #1706 — fetch is a standard web API that Babylon.js code paths depend on. Should be a proper C++ polyfill under Polyfills/, not a Playground-only JS script. See BabylonJS/JsRuntimeHost#98.