Shader graph: Clamp is the Output's default edge, and the sheet's own mode is a constant of its own (#81)

The default of the Edge param and the address mode the shared sheet physically
carries were the same constant; changing the value would have turned every
sentinel with it and given each shader sprite a full copy of its texture.
SHEET_EDGE ('mirror', what TextureLibrary uploads) is now separate from
DEFAULT_EDGE ('clamp'). A graph whose Sample reads nothing but its own pixel
draws with the plain sheet whatever the Edge says, so the common shader costs
no second copy.

Without a shader the quad's coordinates never leave 0..1, and mirror-repeat
reads like clamp within a texel - so clamp is what a sprite shows anyway.

src/viewport/TextureLibrary.ts

@@ -1,6 +1,6 @@
 import { Assets, CanvasSource, Texture } from 'pixi.js';
 import type { FrameRect, TextureRef } from '../core/model/texture';
-import { DEFAULT_EDGE, type EdgeMode } from '../core/shader/nodes';
+import { type EdgeMode, SHEET_EDGE } from '../core/shader/nodes';
 
 const DEFAULT_BASE = '/game-maker/data/textures';
 
@@ -47,10 +47,10 @@ export class TextureLibrary {
    * shared source, so two assets naming the same file with different `smooth` fight over it and
    * the last one drawn wins; upload avoids that by reusing the existing asset for a known file.
    */
-  get(ref: TextureRef, edge: EdgeMode = DEFAULT_EDGE): Texture | null {
+  get(ref: TextureRef, edge: EdgeMode = SHEET_EDGE): Texture | null {
     const sheet = this.load(ref.file);
     if (!sheet) return null;
-    const rect = ref.rect ?? (edge === DEFAULT_EDGE ? null : wholeOf(sheet));
+    const rect = ref.rect ?? (edge === SHEET_EDGE ? null : wholeOf(sheet));
     const texture = rect ? this.crop(ref.file, sheet, rect, edge) : sheet;
     if (!texture) return null;
     const mode = ref.smooth ? 'linear' : 'nearest';
@@ -96,7 +96,7 @@ export class TextureLibrary {
    */
   private crop(file: string, sheet: Texture, rect: FrameRect, edge: EdgeMode): Texture | null {
     if (rect.w <= 0 || rect.h <= 0) return null;
-    const key = edge === DEFAULT_EDGE ? cropKey(file, rect) : `${cropKey(file, rect)}|${edge}`;
+    const key = edge === SHEET_EDGE ? cropKey(file, rect) : `${cropKey(file, rect)}|${edge}`;
     let texture = this.crops.get(key);
     if (!texture) {
       const canvas = new OffscreenCanvas(rect.w, rect.h);