wave E stage 2: renderSvg goes native-first - resvg in-process

Visual-maker SVGs never execute script, so a browser added nothing but a
process. resvg rasterizes in-process now (white background and fitTo
width mirror the old Chrome-page semantics; svgViewport unchanged,
quirks included for parity). The old Chrome path stays verbatim as the
fallback for exotic-SVG parse failures - it was the only path before,
so nothing regresses.

The resvg binary (4MB per platform, one per build) rides the standalone
via the same tracing pattern. Gates 1519/1519; native path pinned
against real SVGs (intrinsic size, viewBox-only 900 default, opts
override, system-font text).

tests/svg-native-render.test.ts

@@ -0,0 +1,69 @@
+/**
+ * Wave E stage 2 (owner 2026-10-10): renderSvg is NATIVE FIRST — resvg
+ * rasterizes in-process; the old Chrome path stays as fallback for
+ * exotic-SVG parse failures. These tests pin the native path against real
+ * SVGs: attribute-sized, viewBox-only (the 900px default), text-bearing
+ * (system fonts), and the canonical single-IDAT PNG form.
+ */
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { realHeadless } from "../src/agent/subagents/headless.ts";
+
+function pngFacts(buf: Buffer): { w: number; h: number; idats: number } {
+	assert.equal(buf.subarray(0, 8).toString("hex"), "89504e470d0a1a0a", "PNG signature");
+	const w = buf.readUInt32BE(16);
+	const h = buf.readUInt32BE(20);
+	let off = 8;
+	let idats = 0;
+	while (off < buf.length) {
+		const len = buf.readUInt32BE(off);
+		const type = buf.subarray(off + 4, off + 8).toString("ascii");
+		if (type === "IDAT") idats++;
+		off += 12 + len;
+	}
+	return { w, h, idats };
+}
+
+describe("native renderSvg (resvg in-process, browser fallback)", () => {
+	it("renders an attribute-sized SVG at its intrinsic size", async () => {
+		const svg =
+			'<svg xmlns="http://www.w3.org/2000/svg" width="240" height="160"><rect width="240" height="160" fill="#3355aa"/></svg>';
+		const png = await realHeadless.renderSvg(svg);
+		const f = pngFacts(png);
+		assert.equal(f.w, 240);
+		assert.equal(f.h, 160);
+		assert.equal(f.idats, 1, "single-IDAT canonical PNG");
+	});
+
+	it("viewBox-only SVG gets the 900px-wide default", async () => {
+		// No width attr anywhere (svgViewport grabs the FIRST width= in the
+		// document — children included — a pre-existing quirk kept for
+		// parity; use a circle so the default branch is exercised).
+		const svg =
+			'<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 300 200"><circle cx="150" cy="100" r="90" fill="#aa5533"/></svg>';
+		const png = await realHeadless.renderSvg(svg);
+		const f = pngFacts(png);
+		assert.equal(f.w, 900, "svgViewport default width");
+		assert.equal(f.h, 600, "aspect preserved from viewBox");
+	});
+
+	it("width/height opts override for viewBox-only SVGs", async () => {
+		const svg =
+			'<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 100"><rect width="400" height="100" fill="#33aa55"/></svg>';
+		const png = await realHeadless.renderSvg(svg, { width: 500, height: 125 });
+		const f = pngFacts(png);
+		assert.equal(f.w, 500);
+	});
+
+	it("text-bearing SVG renders with system fonts (non-trivial output)", async () => {
+		const svg = `<svg xmlns="http://www.w3.org/2000/svg" width="300" height="100">
+  <rect width="300" height="100" fill="#ffffff"/>
+  <text x="150" y="60" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="32" fill="#111111">Harness</text>
+</svg>`;
+		const png = await realHeadless.renderSvg(svg);
+		const f = pngFacts(png);
+		assert.equal(f.w, 300);
+		// A blank 300x100 white PNG is ~300 bytes; glyphs push it well past.
+		assert.ok(png.length > 1000, `text rendered (png ${png.length} bytes > 1000)`);
+	});
+});