fix: clamp SVG rasterization size to prevent memory-exhaustion DoS

[bunlongheng]

Following up on #6 (thanks for the quick merge!) with a small defensive hardening fix.

Problem

rasterizeSvg() in src/skia/image.ts reads an SVG's intrinsic size and passes it straight to MakeSurface():

const [width, height] = readContainerSize(dom, markup);
const surface = kit.MakeSurface(Math.ceil(width), Math.ceil(height));

The size comes from the SVG's own width/height attributes (or viewBox), none of which is bounded by the document's byte length. Because a raster surface costs width * height * 4 bytes, a tiny document can request an enormous allocation:

SVG bytes	Declared size	Surface allocation
~100	8000 x 8000	~256 MB (measured)
~100	20000 x 20000	~1.6 GB
~100	60000 x 60000	~14 GB

SVG is a documented Photo input and flows through SoneImage.decode, so any application that renders SVG bytes/URLs from an untrusted source is exposed to a memory-exhaustion denial of service from a ~100-byte payload. This is uncontrolled resource consumption (CWE-400 / CWE-789).

Fix

Clamp the longest side to 8192 (a common GPU max-texture limit, far above any realistic layout target) and scale the other axis proportionally, so the image still renders correctly - just bounded in resolution. Non-finite or non-positive declared dimensions (malformed width/viewBox) now fall back to the CSS default replaced-element size instead of reaching MakeSurface.

The change is confined to rasterizeSvg's sizing; normal SVGs are unaffected.

Testing

• Added test/image-svg-dos.test.ts: a 60000x60000 and a 50000x25000 SVG now rasterize within the bound (aspect ratio preserved), while a normal 200x120 SVG is unchanged. The DoS test fails on main (unbounded allocation / throw) and passes with this fix.
• Full suite green: 397 passed.

No behavior change for legitimately sized documents; purely a bound on adversarial ones.

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