docs(readme): make thumbnail dynamic — default + latest-capture lookup

- Rename IDrawingTemplate.Thumbnail → DefaultThumbnail (authored fallback)
- Add IGalleryService.GetLatestThumbnailAsync(templateId) for catalog grid
- DrawingCatalog cells show latest user capture when available, else default;
  refresh on ArtworkSavedSignal so progress reflects without a reopen

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Readme.md

@@ -381,7 +381,7 @@ namespace Darkmatter.Core.Contracts.Features.Drawing;
 public interface IDrawingTemplate {
     string Id { get; }
     string DisplayName { get; }
-    Sprite Thumbnail { get; }
+    Sprite DefaultThumbnail { get; }         // authored fallback (used when user has no captures for this template)
     Sprite PaperBackground { get; }
     IReadOnlyList<ShapePieceDTO> Pieces { get; }
     IReadOnlyList<ColorRegionDTO> Regions { get; }
@@ -490,7 +490,12 @@ public interface IGalleryService {
     UniTask<SavedArtworkDTO> SaveAsync(byte[] png, string templateId);
     UniTask<IReadOnlyList<SavedArtworkDTO>> ListAsync();
     UniTask<Texture2D> LoadFullAsync(string artworkId);
+    UniTask<Texture2D> LoadThumbnailAsync(string artworkId);
     UniTask DeleteAsync(string artworkId);
+
+    // Newest captured thumbnail for the given template, or null if none exist.
+    // Catalog cells fall back to IDrawingTemplate.DefaultThumbnail when this returns null.
+    UniTask<Texture2D> GetLatestThumbnailAsync(string templateId);
 }
 
 namespace Darkmatter.Core.Contracts.Services.Capture;
@@ -539,8 +544,10 @@ public readonly struct ArtworkSavedSignal {
 
 ### `DrawingCatalog`
 
-- Loads the catalog manifest (list of available template IDs + thumbnail addresses).
+- Loads the catalog manifest (list of available template IDs).
 - Presents a scrollable grid of thumbnails (Canvas).
+- **Each cell shows the latest captured thumbnail** for that template via `IGalleryService.GetLatestThumbnailAsync(templateId)`. If the user has no captures yet for that template, falls back to `IDrawingTemplate.DefaultThumbnail` (the authored sprite).
+- Subscribes to `ArtworkSavedSignal` — re-fetches the cell's thumbnail for the affected `TemplateId` so the grid reflects user progress without a reopen.
 - On select → fires `DrawingSelectedSignal(templateId)` and unloads the catalog UI.
 
 ### `Paper`
@@ -1305,14 +1312,16 @@ Immutable view of a single drawing's authored data.
 public interface IDrawingTemplate {
     string Id { get; }                              // e.g. "animals/elephant"
     string DisplayName { get; }                     // user-facing
-    Sprite Thumbnail { get; }                       // 256×256 for catalog grid
-    Sprite PaperBackground { get; }                 // composited under artwork
+    Sprite DefaultThumbnail { get; }                // 256×256 authored fallback for the catalog grid
+    Sprite PaperBackground { get; }                 // image under all paper content
     IReadOnlyList<ShapePieceDTO> Pieces { get; }    // for ShapeBuilder
     IReadOnlyList<ColorRegionDTO> Regions { get; }  // for Coloring
 }
 ```
 Implemented by `DrawingTemplateSO` (ScriptableObject) loaded via Addressables.
 
+> The catalog grid shows the latest user-captured thumbnail (via `IGalleryService.GetLatestThumbnailAsync`) when available, falling back to `DefaultThumbnail` when the user hasn't completed this template yet. The template itself stays immutable.
+
 #### `IDrawingTemplateCatalog` *(Core/Contracts/Features/Drawing — planned)*
 Authority on which drawings exist, completion state, and "next" selection.
 ```csharp
@@ -1344,11 +1353,17 @@ Persistent store of saved artwork PNGs.
 public interface IGalleryService {
     UniTask<SavedArtworkDTO> SaveAsync(byte[] png, string templateId);
     UniTask<IReadOnlyList<SavedArtworkDTO>> ListAsync();         // sorted newest first
-    UniTask<Texture2D> LoadFullAsync(string artworkId);    // for fullscreen view
+    UniTask<Texture2D> LoadFullAsync(string artworkId);          // fullscreen view
     UniTask<Texture2D> LoadThumbnailAsync(string artworkId);
     UniTask DeleteAsync(string artworkId);
+
+    // Newest captured thumbnail for the given template, or null if the user has
+    // no captures for it. Used by the catalog grid; null → caller falls back to
+    // IDrawingTemplate.DefaultThumbnail.
+    UniTask<Texture2D> GetLatestThumbnailAsync(string templateId);
 }
 ```
+For v1 the latest-thumbnail lookup can list-and-filter (tens of templates max). Add an in-memory `Dictionary<templateId, latestArtworkId>` cache later if perf becomes a concern.
 
 #### `ICaptureService` *(Core/Contracts/Services/Capture — planned)*
 Snapshots the paper RT to a PNG blob. No arguments — dimensions and content come from `IPaperRig.Surface`.