fix(ascii-stroke): narrow SVG root via instanceof, drop unsafe cast

`doc.documentElement` is typed HTMLElement, so `as SVGSVGElement` tripped
ts(2352) and was blocking the pre-push astro-check gate (pre-existing,
unrelated to README). Replace the tagName string guard with
`instanceof SVGSVGElement` — it narrows root properly (and rejects
<parsererror>/non-SVG roots), so importNode now returns SVGSVGElement and
no cast is needed. No runtime behavior lost; the gate goes green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: chodaict

src/lib/ascii-stroke-dom.ts

@@ -58,8 +58,10 @@ export function sampleSvgStrokes(
   if (typeof document === 'undefined') return null;
 
   const doc = new DOMParser().parseFromString(svgText, 'image/svg+xml');
+  // instanceof narrows root to SVGSVGElement for the rest of the function (and
+  // rejects <parsererror>/non-SVG roots), so no downstream cast is needed.
   const root = doc.documentElement;
-  if (!root || root.nodeName === 'parsererror' || root.tagName.toLowerCase() !== 'svg') return null;
+  if (!(root instanceof SVGSVGElement)) return null;
 
   // Resolve the user-space box: explicit viewBox, else width/height.
   const vb = root.getAttribute('viewBox');
@@ -92,7 +94,7 @@ export function sampleSvgStrokes(
     'style',
     'position:absolute;left:-99999px;top:0;width:0;height:0;overflow:hidden',
   );
-  const mounted = document.importNode(root, true) as SVGSVGElement;
+  const mounted = document.importNode(root, true);
   mounted.setAttribute('width', String(vbW));
   mounted.setAttribute('height', String(vbH));
   mounted.setAttribute('preserveAspectRatio', 'xMidYMid meet');