Add WebGL book scene checkpoint

public/assets/webgl/README.md

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+WebGL Scene Assets
+==================
+
+- `wood_table_diff_1k.jpg`
+  - Source: Poly Haven, "Wood Table"
+  - URL: https://polyhaven.com/a/wood_table
+  - Author: Dimitrios Savva
+  - License: CC0
+
+- `book/open-book-poly-pizza.glb`
+  - Source: Poly Pizza, "open book"
+  - URL: https://poly.pizza/m/4WPcl72i1_S
+  - Author: Justin Randall
+  - License: Creative Commons Attribution 3.0
+  - Notes: Real authored open-book GLB used as the visible book model.
+
+- `book/open-book-poly-pizza-preview.jpg`
+  - Source: Poly Pizza, "open book" preview image
+  - URL: https://poly.pizza/m/4WPcl72i1_S
+  - Author: Justin Randall
+  - License: Creative Commons Attribution 3.0
+
+Candidate model found during follow-up search:
+
+- "Old Magical Book" by Akiko.Tomiyoshi on Sketchfab
+  - URL: https://sketchfab.com/3d-models/old-magical-book-326cf7653c7c4ec19d2672f5a7a33578
+  - License: Creative Commons Attribution
+  - Notes: The model description says it has page-flipping animation using bone and lattice modifiers.
+  - Blocker: Sketchfab's download API requires authenticated credentials, so it was not pulled into the repository automatically.
+
+Candidate model checked and rejected for automated import:
+
+- "Rigged book" by Jissse on Blend Swap
+  - URL: https://blendswap.com/blend/26504
+  - License: CC0
+  - Notes: The page lists a Blender rig with controllable pages.
+  - Blocker: The file download requires signing in; the unauthenticated response is a download page, not model bytes.