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.

tools/e2e/28-shader.mjs

@@ -614,6 +614,24 @@ export async function shader(t) {
     colourDistance(mirror.left, RIGHT) < 12 && colourDistance(mirror.right, RIGHT) < 12,
     mirror,
   );
+  // u + 0.5 cannot tell Clamp from Mirror: both hold green past the border. u + 0.8 puts the near
+  // half on 1.6, where Mirror is already back in the far half and Clamp still holds the last texel.
+  const scrolledFar = async (outParams) => {
+    await sampleAt(
+      [node('far', 'ShaderAdd', { b: 0.8 }, 260, 0)],
+      [edge('uv', 'u', 'far', 'a'), edge('far', 'result', 'sample', 'u')],
+      outParams,
+    );
+    await sleep(400);
+    return pixel(0.3, -0.3);
+  };
+  const byDefault = await scrolledFar({});
+  const byMirror = await scrolledFar({ edge: 'mirror' });
+  check(
+    'with nothing set on the Output the edge is Clamp: at u + 0.8 the border texel goes on, where Mirror shows the near half again',
+    colourDistance(byDefault, RIGHT) < 12 && colourDistance(byMirror, LEFT) < 12,
+    { byDefault, byMirror },
+  );
   await evaluate("__gm.showTab('shader')");
   await sleep(300);
   await shot('28-shader-scroll-graph');