Shader graph: alpha input on Output, Texture alpha node (#70)

Output gains a number input `alpha`; unconnected it reads the texel's
alpha, like `color` reads the texel. The epilogue premultiplies with the
resolved alpha. New `Texture alpha` node exposes texel.a for maths.

PortDef.inline=false keeps the alpha port without an inline field: every
number port is inline by default, and a typed default of 0 would have made
every shaded sprite invisible. The compiler's texel fallback is typed:
colour ports get rgb, number ports get texel.a.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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, 699 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 703 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