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.

README.md

@@ -37,7 +37,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 784 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 785 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs