3D 棋盘按战场显示官方场景（沙地等 11 个战场），场景由本地提取脚本从赛季场景包导出，没有时回退现有金属棋盘 (#532)

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+# DESIGN §30 — 0.2.5
+
+Part of [DESIGN.md](../DESIGN.md). This chapter records the changes after the 0.2.4 release.
+
+## 30. After the 0.2.4 release
+
+### 30.1 Official battlefield scenes (#532)
+
+Report: the web board draws the metal theme for every battlefield, which looks unlike the official client; the
+reporter asked for the sand theme.
+
+Findings (2026-10-11; evidence: the official hot-update lists and asset indexes, read from the CDN, and the scene
+bundles of version `26-04-13-12-31-42_e9b560`):
+
+- The official board is not assembled at run time. Every battlefield is a prebuilt scene bundle, shipped only while
+  the season runs — §15 said otherwise from an off-season list and is corrected.
+- Which art a battlefield uses is the dependency list of its scene bundle: the sand theme for act1 m01–m07 and act2
+  m02 / m04, `map_lakegarden` + `map_syracuse_metro` for act2 m01, `map_kxmr` + `map_lm_center` for act2 m03. The
+  same in the 上半 version of 2025-11 and the 下半 versions of 2026-03 / 04. No battlefield uses the metal theme.
+- The sand theme bundle holds no mesh because the models are baked into each scene ("Combined Mesh (root: scene)").
+- A scene's three sub-graphics `HIDDEN@1…3` are the left normal field, the right normal field and the boss field.
+
+Change: `tools/local-extract/scenes.py` exports a battlefield's scene (meshes in board space, materials, key light,
+lightmap pages with the baked shadow mask in alpha, albedo textures) to `public/assets/local/scene/<stageId>/`;
+`extract.py --only scene` runs it, reading the scene bundles from the client, `--scenes <dir>` or
+`.cache/official-scenes/*`; `tools/local-extract/fetch_scenes.py` downloads the in-season bundles from the official
+CDN (md5-checked against that version's hot-update list). The 3D board draws a stage's scene when the local-art
+manifest lists it (`board3d/load.js loadScenePack`, `scene.js setScenePack`); the procedural board stays the fallback
+and still draws the devices, gates, field edges and terrain overlays. All eleven battlefields extract and draw.
+
+No gameplay change: the scene is art under the same grid, camera and unit layer; golden results are untouched.
+
+**[ASSUMED]** the light gains `SCENE_LIGHT` and the RGBM reading of the lightmap pages (§15); the approximations are
+listed there. Not done: the scene's normal / metallic / emission maps, map effects, sky box and post-processing; the
+tile graphics a level switches at run time (mire, tide) still use the remake's overlays.
+
+Tests: `test/render/board3d-scene.test.js` (the scene table against data/stages.json, board-space conversion, chunk
+layout shared by scenes.py and load.js, sub-graphic groups, RGBM decode, the loader with a fake store, BoardScene with
+a stub renderer: scene on / areas / fallback, and the extracted files when present).