Support SVG sources (closes #68)

The fetcher accepts image/svg+xml; internal/magic detects SVG. orig of
an SVG at the orig size is the SVG unchanged, cached, with a
Content-Security-Policy allowing only its styles and data: images; at
another size, ErrSVGNotOriginalSize (400). Any other format renders it
at the least size covering the output, at most 8192 on a side, so it
stays sharp. Rendering refuses an SVG with a side over 8192 times the
other (ErrSVGTooThin, 415), and the orig size of one above 8192 on a
side (ErrSVGTooLargeForOrigSize, 400). The cache reads up to 1024 first
bytes to find a file's format. Tests pinning the SVG refusal now expect
SVG. Render time and memory are unbounded: P1 in TODO.md until the
owner decides on #68.

Model: opus-5-5

internal/imgcache/source_content_internal_test.go

@@ -216,8 +216,6 @@ func TestSource_RefusesFileOfNoSourceFormat(t *testing.T) {
 		wantErr error
 	}{
 		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
-		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			magic.ErrUnknownFormat},
 		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
 		{"empty file", nil, io.EOF},
 	}