Add: windows mvp - transparent bugs not fixed

docs/SPRITE_PACK_SCHEMA.md

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+# Sprite Pack Manifest Schema (MVP)
+
+Path: `<pack_dir>/manifest.json`
+
+## Required Fields
+
+- `id` (string): unique sprite pack ID.
+- `version` (string): schema/pack version label.
+- `image` (string): atlas image filename, e.g. `atlas.png`.
+- `frame_width` (u32): frame width in pixels.
+- `frame_height` (u32): frame height in pixels.
+- `animations` (array): list of animation definitions.
+- `anchor` (object): sprite anchor/pivot.
+
+## Animation Definition
+
+- `name` (string)
+- `fps` (u16)
+- `frames` (array of u32 frame indices)
+- `one_shot` (optional bool)
+
+## Anchor Object
+
+- `x` (f32)
+- `y` (f32)
+
+## Example
+
+```json
+{
+  "id": "default",
+  "version": "1",
+  "image": "atlas.png",
+  "frame_width": 64,
+  "frame_height": 64,
+  "animations": [
+    { "name": "idle", "fps": 8, "frames": [0, 1, 2, 3] },
+    { "name": "success", "fps": 12, "frames": [20, 21, 22], "one_shot": true }
+  ],
+  "anchor": { "x": 0.5, "y": 1.0 }
+}
+```
+
+## Runtime Fallback Behavior
+
+- The selected pack is attempted first.
+- If selected pack is missing/invalid, frontend falls back to `default`.
+- If `default` is missing/invalid, loading fails with error.
+
+## Alpha Handling
+
+- If the source image already has an alpha channel, runtime uses it directly.
+- If the source image is RGB-only, runtime can apply chroma-key conversion:
+  - default key color: `#FF00FF`
+  - matching pixels become transparent.
+- Manifest optional fields:
+  - `chroma_key_color` (string `#RRGGBB`)
+  - `chroma_key_enabled` (bool)
+
+## Grid Derivation Rule
+
+- Runtime derives atlas grid from image dimensions:
+  - `columns = image_width / frame_width`
+  - `rows = image_height / frame_height`
+- Image dimensions must be divisible by frame dimensions.
+- Every animation frame index must be `< columns * rows`.