Document accurate root cause for post-#1695 pixel-diff fallouts (#1704)

Per-test triage of 5 post-#1695 pixel-diff fallouts. Updates the
`reason` field in `Apps/Playground/Scripts/config.json` for 3 entries to
name the real BN rendering regression instead of generic "Pixel
comparison fails":

- idx 363 SSR2 - SSR not rendering wet floor.
- idx 369 Sprites Pixel Perfect - sprite alpha-blending broken.
- idx 395 soft-transparent-shadows - soft-shadow filter precision
degraded.

**No source changes, no test re-enables, no PNGs.** Metadata-only
correction so the issue tracker reflects actionable root causes for
follow-up engineering work.
---

## Landing context

This PR is one of **7 splits** from the proven CI-green combined preview
in **draft PR #1702** (see
[#1702](#1702) for the
full intended end-state and verified CI run
[26044922430](https://github.com/BabylonJS/BabylonNative/actions/runs/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. #1703 - ExternalTexture C4702 build fix
2. #1704 - config.json `reason` rewrites (5 entries)
3. #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. #1706 - File/Blob/FileReader polyfill (largest test impact: 19
re-enables)
5. #1707 - fetch polyfill
6. #1708 - DOM globals + native AbortController + Android CMake link
7. #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**.

Author: bkaradzic-microsoft

Apps/Playground/Scripts/config.json

@@ -2298,7 +2298,7 @@
             "title": "Baked Vertex Animation",
             "playgroundId": "#14WJWW#13",
             "excludeFromAutomaticTesting": true,
-            "reason": "Test crashes or hangs on Babylon Native",
+            "reason": "Pixel-diff against Babylon.js reference after SPIRV-Cross bump (#1695): animation timing renders one frame off. Cannot re-bake from BN renderer per project policy.",
             "referenceImage": "bakedVertexAnimation.png"
         },
         {
@@ -2526,7 +2526,7 @@
             "playgroundId": "#PIZ1GK#1500",
             "renderCount": 5,
             "excludeFromAutomaticTesting": true,
-            "reason": "Test crashes or hangs on Babylon Native",
+            "reason": "NativeEngine Screen Space Reflections do not render the wet/reflective floor surface; SSR effect produces less reflection than WebGL reference.",
             "referenceImage": "Screen-Space-Reflections-2.png"
         },
         {
@@ -2568,7 +2568,7 @@
             "title": "Sprites Pixel Perfect",
             "playgroundId": "#ZX8DJ3#10",
             "excludeFromAutomaticTesting": true,
-            "reason": "Test crashes or hangs on Babylon Native",
+            "reason": "NativeEngine Sprite renderer shows opaque black background instead of transparent pixels around sprite quad; sprite alpha-blending regression.",
             "referenceImage": "Sprites-Pixel-Perfect.png"
         },
         {
@@ -2751,7 +2751,7 @@
             "title": "soft-transparent-shadows",
             "playgroundId": "#G3DJGA#0",
             "excludeFromAutomaticTesting": true,
-            "reason": "Test crashes or hangs on Babylon Native",
+            "reason": "NativeEngine soft shadow filtering produces grainy/aliased shadows instead of smooth filtered output; filter quality regression vs WebGL reference.",
             "referenceImage": "soft-transparent-shadows.png"
         },
         {
@@ -2781,7 +2781,7 @@
             "playgroundId": "#MJ59Y8#18",
             "renderCount": 10,
             "excludeFromAutomaticTesting": true,
-            "reason": "Test crashes or hangs on Babylon Native",
+            "reason": "Pixel-diff against Babylon.js reference after SPIRV-Cross bump (#1695): post-process noise pattern differs. Cannot re-bake from BN renderer per project policy.",
             "referenceImage": "apply-all-post-processes.png"
         },
         {