chore: draw the toolbar icons in-tree with make icons (closes #378)

script/lib/icons.js draws the mark from geometry read back out of the
committed PNGs, since the coordinates were never recorded, and writes each
PNG with node's own zlib; `make icons` runs it. tests/icons.test.js compares
what it draws with icons/ byte for byte.

That test is red on purpose. Every pixel matches at all four sizes, but
node 22's bundled zlib compresses at level 9 differently from the stock
zlib 1.3.1 that made the committed files, so the IDAT bytes differ. How to
settle that is the owner's call, asked on the issue. TODO.md is updated
with the landing commit.

build.js copies the manifests from MANIFEST_SOURCES instead of naming the
two paths a second time.

Model: opus-5-5

script/lib/icons.js

@@ -0,0 +1,147 @@
+// Draws the toolbar icon and writes it to every file manifest/chrome.json
+// declares under "icons". Run by `make icons`; tests/icons.test.js checks that
+// the committed files are exactly what this draws.
+//
+// The icon is a dark-navy rounded square carrying a teal triangle with a
+// smaller triangle cut out of it. Every coordinate below is a fraction of the
+// icon's side, so one drawing serves every size. A pixel's colour is the
+// average of an 8x8 grid of samples, one at the centre of each cell, which
+// smooths the edges.
+//
+// The PNG is written here rather than by a library: RGBA at 8 bits per
+// channel, filter type 0 (none) on every row, one IDAT chunk compressed by
+// node's zlib at level 9. Changing any of that changes the bytes.
+
+"use strict";
+
+const fs = require("fs");
+const path = require("path");
+const zlib = require("zlib");
+
+const { icons } = require("../../manifest/chrome.json");
+
+const NAVY = [0x10, 0x1a, 0x2e];
+const TEAL = [0x35, 0xe0, 0xc2];
+const CORNER_RADIUS = 0.22;
+const OUTER_TRIANGLE = [
+    [0.5, 0.15],
+    [0.115, 0.855],
+    [0.885, 0.855],
+];
+const INNER_TRIANGLE = [
+    [0.5, 0.4],
+    [0.29, 0.7],
+    [0.71, 0.7],
+];
+const SAMPLES_PER_SIDE = 8;
+
+const PNG_SIGNATURE = Buffer.from([
+    0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
+]);
+
+// Points are in pixels from the icon's top-left corner.
+function insideRoundedSquare(x, y, size) {
+    const r = CORNER_RADIUS * size;
+    // How far the point is past the start of a corner's curve, on each axis.
+    const dx = Math.max(r - x, 0, x - (size - r));
+    const dy = Math.max(r - y, 0, y - (size - r));
+    return dx * dx + dy * dy <= r * r;
+}
+
+// Inside or on an edge: the point is not on opposite sides of two edges.
+function insideTriangle(x, y, [a, b, c]) {
+    const side = (p, q) =>
+        (q[0] - p[0]) * (y - p[1]) - (q[1] - p[1]) * (x - p[0]);
+    const sides = [side(a, b), side(b, c), side(c, a)];
+    return !(sides.some((s) => s < 0) && sides.some((s) => s > 0));
+}
+
+// Rounds to the nearest integer and a half to the even neighbour, as the
+// committed icons were made. Math.round takes a half up, which would change
+// some pixels by one.
+function roundHalfToEven(v) {
+    const down = Math.floor(v);
+    if (v - down !== 0.5) {
+        return Math.round(v);
+    }
+    return down % 2 === 0 ? down : down + 1;
+}
+
+// The RGBA bytes of the pixel whose top-left corner is (px, py).
+function pixel(px, py, size, outer, inner) {
+    let inSquare = 0;
+    let inTeal = 0;
+    for (let j = 0; j < SAMPLES_PER_SIDE; j++) {
+        const y = py + (j + 0.5) / SAMPLES_PER_SIDE;
+        for (let i = 0; i < SAMPLES_PER_SIDE; i++) {
+            const x = px + (i + 0.5) / SAMPLES_PER_SIDE;
+            if (!insideRoundedSquare(x, y, size)) {
+                continue;
+            }
+            inSquare++;
+            if (insideTriangle(x, y, outer) && !insideTriangle(x, y, inner)) {
+                inTeal++;
+            }
+        }
+    }
+    if (inSquare === 0) {
+        return [0, 0, 0, 0];
+    }
+    const colour = NAVY.map((navy, k) =>
+        roundHalfToEven(navy + ((TEAL[k] - navy) * inTeal) / inSquare),
+    );
+    const alpha = roundHalfToEven((255 * inSquare) / SAMPLES_PER_SIDE ** 2);
+    return [...colour, alpha];
+}
+
+// A PNG chunk: the data's length, the type, the data, then the CRC-32 of the
+// type and the data.
+function chunk(type, data) {
+    const typeAndData = Buffer.concat([Buffer.from(type, "ascii"), data]);
+    const length = Buffer.alloc(4);
+    length.writeUInt32BE(data.length);
+    const crc = Buffer.alloc(4);
+    crc.writeUInt32BE(zlib.crc32(typeAndData));
+    return Buffer.concat([length, typeAndData, crc]);
+}
+
+// The complete PNG file for the icon at `size` pixels square.
+function drawIcon(size) {
+    const toPixels = (corners) => corners.map(([x, y]) => [x * size, y * size]);
+    const outer = toPixels(OUTER_TRIANGLE);
+    const inner = toPixels(INNER_TRIANGLE);
+
+    const rows = [];
+    for (let py = 0; py < size; py++) {
+        const row = [0]; // filter type 0: the row's bytes are stored as they are
+        for (let px = 0; px < size; px++) {
+            row.push(...pixel(px, py, size, outer, inner));
+        }
+        rows.push(Buffer.from(row));
+    }
+
+    const header = Buffer.alloc(13); // compression, filter, interlace: all 0
+    header.writeUInt32BE(size, 0); // width
+    header.writeUInt32BE(size, 4); // height
+    header[8] = 8; // bits per channel
+    header[9] = 6; // colour type: RGBA
+
+    return Buffer.concat([
+        PNG_SIGNATURE,
+        chunk("IHDR", header),
+        chunk("IDAT", zlib.deflateSync(Buffer.concat(rows), { level: 9 })),
+        chunk("IEND", Buffer.alloc(0)),
+    ]);
+}
+
+if (require.main === module) {
+    for (const [size, file] of Object.entries(icons)) {
+        fs.writeFileSync(
+            path.join(__dirname, "..", "..", file),
+            drawIcon(Number(size)),
+        );
+        console.log(`icons: wrote ${file}`);
+    }
+}
+
+module.exports = { drawIcon };