test(thumbhash): dither the fixtures so no DCT coefficient is degenerate

The gradient fixtures are smooth analytic ramps, and alpha.png's alpha channel varied
along x only. Both make whole families of DCT coefficients mathematically zero: 13 of
alpha.png's 38 AC nibbles, and at least one in every other ramp fixture.

A zero coefficient normalizes to exactly the 0.5 midpoint, i.e. 15*f = 7.5, so which of
nibble 7 or 8 it quantizes to is decided by ~1e-16 of float rounding noise. Those nibbles
are therefore unstable across any two summation orders -- the vendored JS and the Go port
already disagreed on solid.png for this reason -- and they carry no signal, so a bug that
transposed two of them would be invisible.

A deterministic +/-4 dither, plus an alpha ramp that varies in x and y, leaves every
coefficient at least 4.8e-5 from the tie boundary: ~1e9 times the observed inter-
implementation noise. solid.png and tiny.png regenerate byte-identical and keep their
existing carve-outs.

core/artwork/thumbhash/testdata/gen_fixtures.mjs

@@ -39,7 +39,20 @@ const png = (w, h, rgba) => {
   ])
 }
 
+// A ramp has exactly-zero DCT coefficients, and a zero coefficient's nibble is decided by float
+// rounding noise, so no two summation orders agree on it. Dither gives every coefficient signal.
+let seed = 0
+const dither = () => {
+  seed ^= seed << 13
+  seed ^= seed >>> 17
+  seed ^= seed << 5
+  seed >>>= 0
+  return (seed % 9) - 4
+}
+const shade = (v) => Math.max(0, Math.min(255, v + dither()))
+
 const make = (w, h, fn) => {
+  seed = 0x9e3779b9
   const rgba = new Uint8Array(w * h * 4)
   for (let y = 0; y < h; y++)
     for (let x = 0; x < w; x++) fn(rgba, (y * w + x) * 4, x, y, w, h)
@@ -47,9 +60,9 @@ const make = (w, h, fn) => {
 }
 
 const gradient = (rgba, i, x, y, w, h) => {
-  rgba[i] = Math.floor((255 * x) / w)
-  rgba[i + 1] = Math.floor((255 * y) / h)
-  rgba[i + 2] = Math.floor((255 * (x + y)) / (w + h))
+  rgba[i] = shade(Math.floor((255 * x) / w))
+  rgba[i + 1] = shade(Math.floor((255 * y) / h))
+  rgba[i + 2] = shade(Math.floor((255 * (x + y)) / (w + h)))
   rgba[i + 3] = 255
 }
 const solid = (rgba, i) => {
@@ -61,10 +74,10 @@ const solid = (rgba, i) => {
 // RGB must vary with position too: a constant color composited over its own average cancels to a
 // flat L/P/Q (the bug this fixture exists to catch), so pair a color gradient with the alpha ramp.
 const alphaRamp = (rgba, i, x, y, w, h) => {
-  rgba[i] = Math.floor((255 * x) / w)
-  rgba[i + 1] = Math.floor((255 * y) / h)
-  rgba[i + 2] = Math.floor((255 * (x + y)) / (w + h))
-  rgba[i + 3] = Math.floor((255 * x) / w)
+  rgba[i] = shade(Math.floor((255 * x) / w))
+  rgba[i + 1] = shade(Math.floor((255 * y) / h))
+  rgba[i + 2] = shade(Math.floor((255 * (x + y)) / (w + h)))
+  rgba[i + 3] = shade(Math.floor((255 * (x + 2 * y)) / (w + 2 * h)))
 }
 
 const out = new URL('.', import.meta.url).pathname

core/artwork/thumbhash/testdata/golden.json

@@ -1,8 +1,8 @@
 {
-  "alpha.png": "JFiKBQw3s4ewiId3eBtPOfuEgHd4iIh4eA==",
-  "landscape.png": "3wcOFJpwd3dxd3eHh3ePgAj4hw==",
-  "portrait.png": "3/cNFBpxB4d3d3d4d3eAjwj3eA==",
+  "alpha.png": "JOiFBQ4nkIexh3p4iA8uB+lYhIeAh3d4dw==",
+  "landscape.png": "3wcOFJpwh4eBh3d4iIePgAj3hw==",
+  "portrait.png": "3/cNFBqBB4iId4d3d4iAjwj4hw==",
   "solid.png": "HoUBBwB4eHeHd3hweId3h3h4B2+Ih4gA",
-  "square.png": "HwgOBxpwd4dwd3h3h3eHd3d3+PiIgI8H",
+  "square.png": "H/gNBxpwh4dwd3eIiHd3iHeHeJ+dcH8I",
   "tiny.png": "HoU9tx4I9wiIh4hwj3CI+AiIcH/494cP"
 }