Textures: Linear/Nearest labels; prove the filter pass does not resample pixel art (#68)

Dropdown labels are now Linear (default) and Nearest; the stored values
linear/nearest are unchanged, old project files load as before.

e2e 12c-ii measures in Play at camera zoom 1.37 (screen scale 1.434): a
Nearest edge sampled across seven pixels is byte-identical with and without
a neutral Adjust colors shader, and brightness 0.5 on the same path darkens,
so the pass really runs there. New hook __gm.gameRect(id).

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

tools/e2e/12-textures.mjs

@@ -40,7 +40,7 @@ const PASTE_SPARE = `(async () => {
 
 export async function textures(t) {
   const { check, sleep, json, evaluate, shot, mutate, press } = t;
-  const { api, click, clickText, doubleClick, locate, undo } = t;
+  const { api, click, clickText, doubleClick, locate, undo, play, stop } = t;
 
   // The previous step leaves a picker open and a text field focused, and paste is deliberately
   // ignored while an input has focus, so that Ctrl+V inside a field still edits text.
@@ -170,7 +170,7 @@ export async function textures(t) {
     const hard = await sample('nearest');
     const pure = (c) => colourDistance(c, '#e63c3c') < 60 || colourDistance(c, '#3cc85a') < 60;
     check(
-      'pixel-art filtering keeps the border between two quadrants hard',
+      'nearest filtering keeps the border between two quadrants hard',
       hard.pixels.every(pure),
       JSON.stringify(hard),
     );
@@ -180,11 +180,67 @@ export async function textures(t) {
     // a working switch drifts 425, a hung one 0, and 30 even if every channel were off by two.
     const drift = hard.pixels.reduce((sum, c, i) => sum + colourDistance(c, soft.pixels[i]), 0);
     check(
-      'switching the filter back to smooth blends the border without a reload',
+      'switching the filter back to linear blends the border without a reload',
       soft.pixels.some((c) => !pure(c)) && drift > 150,
       JSON.stringify({ drift, nearest: hard, linear: soft }),
     );
     await shot('33-texture-filter-smooth');
+
+    // 12c-ii. A shader is a filter pass: the sprite goes into an intermediate texture and back. At a
+    // camera zoom that puts the picture on a fractional pixel grid, that copy must not resample the
+    // nearest-filtered border. Measured in play, where the camera's zoom is the world scale; one play
+    // per setting, since the play world is built from the project when it starts.
+    await mutate(
+      `__gm.state.project.assets.find((a) => a.id === ${JSON.stringify(texture.id)}).props.filter = 'nearest'`,
+    );
+    const placed = await json(
+      "(() => { const o = __gm.state.activeScene.objects.find((o) => o.id === 'otex'); return { x: o.position.x, y: o.position.y }; })()",
+    );
+    await mutate(`
+      const o = __gm.state.activeScene.objects.find((o) => o.id === 'otex');
+      o.position.x = 0; o.position.y = 0;
+      __gm.state.project.assets.find((a) => a.id === 'cam').props.zoom = 1.37`);
+    const played = async (patch) => {
+      await mutate(
+        `Object.assign(__gm.state.project.assets.find((a) => a.id === 'tex-sprite').props, ${JSON.stringify(patch)})`,
+      );
+      await play();
+      await sleep(500);
+      const rect = await json("__gm.gameRect('otex')");
+      const y = Math.round(rect.y - rect.h * 0.25);
+      const pixels = [];
+      for (let dx = -3; dx <= 3; dx++)
+        pixels.push(await json(`__gm.pixelAt(${Math.round(rect.x + dx)}, ${y})`));
+      await stop();
+      return { pixels, scale: rect.w / 320 };
+    };
+    const bare = await played({ shader: '' });
+    const filtered = await played({ shader: 'adjust-colors' });
+    const darkened = await played({ shader: 'adjust-colors', brightness: 0.5 });
+    check(
+      'at a fractional camera zoom the nearest border is still hard in play',
+      Number.isInteger(bare.scale) === false && bare.pixels.every(pure),
+      JSON.stringify(bare),
+    );
+    check(
+      'a neutral shader draws the very same pixels there: the filter pass does not resample',
+      JSON.stringify(filtered.pixels) === JSON.stringify(bare.pixels),
+      JSON.stringify({ bare: bare.pixels, filtered: filtered.pixels }),
+    );
+    // Without this, "the same" could also mean the shader never ran on that path.
+    check(
+      'and the shader does run on that path: brightness 0.5 darkens every one of them',
+      darkened.pixels.every((c, i) => colourDistance(c, bare.pixels[i]) > 100 && !pure(c)),
+      JSON.stringify(darkened),
+    );
+    await mutate(`
+      const o = __gm.state.activeScene.objects.find((o) => o.id === 'otex');
+      o.position.x = ${placed.x}; o.position.y = ${placed.y};
+      __gm.state.project.assets.find((a) => a.id === 'cam').props.zoom = 1;
+      const sprite = __gm.state.project.assets.find((a) => a.id === 'tex-sprite');
+      sprite.props.shader = ''; delete sprite.props.brightness;
+      __gm.state.project.assets.find((a) => a.id === ${JSON.stringify(texture.id)}).props.filter = 'linear'`);
+    await sleep(400);
     await mutate(enlarge(160, 120));
     await sleep(400);