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.

tests/core/shaders.test.ts

@@ -36,6 +36,8 @@ import {
 import { inlineInputs } from '../../src/core/graph/NodeRegistry';
 import { compileShader, shaderFingerprint } from '../../src/core/shader/compile';
 import {
+  DEFAULT_EDGE,
+  EDGE_PARAM,
   SHADER_NODES,
   SHADER_NUMBER,
   SHADER_OUTPUT,
@@ -47,6 +49,7 @@ import {
   SHADER_UV,
   ShaderOutput,
   ShaderSample,
+  SHEET_EDGE,
 } from '../../src/core/shader/nodes';
 import { deserializeProject } from '../../src/core/store/serialize';
 import { paletteEntries } from '../../src/editor/graph/paletteEntries';
@@ -414,16 +417,29 @@ describe('compileShader', () => {
     const s = shaderEditor(f, 'Idle');
     s.add(SHADER_TIME);
     const out = s.add(SHADER_OUTPUT);
-    let compiled = compileShader(s.asset().graph).compiled!;
-    expect(compiled).toMatchObject({ time: false, edge: 'mirror' });
-    expect(compiled.fragment).not.toContain('uTime');
     const params = s.asset().graph.nodes.find((n) => n.id === out)!.params;
-    params.edge = 'clamp';
-    expect(compileShader(s.asset().graph).compiled!.edge).toBe('clamp');
+    // A new Output carries the default, so a graph never silently changes when the default does.
+    expect(params[EDGE_PARAM]).toBe('clamp');
+    let compiled = compileShader(s.asset().graph).compiled!;
+    expect(compiled).toMatchObject({ time: false, edge: SHEET_EDGE });
+    expect(compiled.fragment).not.toContain('uTime');
+    // Nothing reads past the border here, so no mode can show – and none costs a second picture.
+    params[EDGE_PARAM] = 'repeat';
+    expect(compileShader(s.asset().graph).compiled!.edge).toBe(SHEET_EDGE);
+    const uv = s.add(SHADER_UV);
+    const shift = s.add('ShaderAdd', { b: 0.5 });
+    const sample = s.add(SHADER_SAMPLE);
+    s.link(uv, 'u', shift, 'a');
+    s.link(shift, 'result', sample, 'u');
+    s.link(sample, 'color', out, 'color');
+    expect(compileShader(s.asset().graph).compiled!.edge).toBe('repeat');
     // A value the dropdown never offers reads as the default rather than as a mode.
-    params.edge = 'sideways';
+    params[EDGE_PARAM] = 'sideways';
     compiled = compileShader(s.asset().graph).compiled!;
-    expect(compiled.edge).toBe('mirror');
+    expect(compiled.edge).toBe(DEFAULT_EDGE);
+    // A graph from before the setting existed has no value at all and takes the default too (#81).
+    delete params[EDGE_PARAM];
+    expect(compileShader(s.asset().graph).compiled!.edge).toBe('clamp');
   });
 
   it('writes Fract, Step and Floor as the GLSL calls, per channel for a colour', () => {