use TextEncoder/TextDecoder for UTF-8

Every supported runtime has them. The hand-written decoder accepted
overlong sequences and encoded surrogates, which TextDecoder replaces with
U+FFFD as the encoding standard specifies, and it's faster on large SVGs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ljnxm1Qjrocq1izGajLx8J

Author: matmen

src/util/utf8.ts

@@ -1,60 +1,17 @@
-// UTF-8 helpers, so the library doesn't depend on TextEncoder/TextDecoder being available
+// TextEncoder and TextDecoder are web APIs, not part of the es2022 lib: declare the parts used here
+declare const TextEncoder: { new(): { encode(text: string): Uint8Array } };
+declare const TextDecoder: { new(): { decode(bytes: Uint8Array): string } };
 
-export function encode(text: string): Uint8Array {
-    const out: number[] = [];
-
-    for (const char of text) {
-        const c = char.codePointAt(0) as number;
-        if (c < 0x80) out.push(c);
-        else if (c < 0x800) out.push(0xc0 | c >> 6, 0x80 | c & 63);
-        else if (c < 0x10000) out.push(0xe0 | c >> 12, 0x80 | c >> 6 & 63, 0x80 | c & 63);
-        else out.push(0xf0 | c >> 18, 0x80 | c >> 12 & 63, 0x80 | c >> 6 & 63, 0x80 | c & 63);
-    }
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
 
-    return Uint8Array.from(out);
+export function encode(text: string): Uint8Array {
+    return encoder.encode(text);
 }
 
 /**
  * Decodes UTF-8, replacing invalid sequences with U+FFFD and skipping a leading byte order mark
  */
 export function decode(bytes: Uint8Array): string {
-    let out = '';
-    let i = bytes[0] === 0xef && bytes[1] === 0xbb && bytes[2] === 0xbf ? 3 : 0;
-
-    while (i < bytes.length) {
-        const b = bytes[i++];
-        let c: number, extra: number;
-
-        if (b < 0x80) {
-            c = b;
-            extra = 0;
-        } else if (b >= 0xc2 && b < 0xe0) {
-            c = b & 31;
-            extra = 1;
-        } else if (b >= 0xe0 && b < 0xf0) {
-            c = b & 15;
-            extra = 2;
-        } else if (b >= 0xf0 && b < 0xf5) {
-            c = b & 7;
-            extra = 3;
-        } else {
-            out += '�';
-            continue;
-        }
-
-        let valid = true;
-        for (let k = 0; k < extra; k++) {
-            const next = bytes[i];
-            if (next === undefined || (next & 0xc0) !== 0x80) {
-                valid = false;
-                break;
-            }
-            c = c << 6 | next & 63;
-            i++;
-        }
-
-        out += valid && c <= 0x10ffff ? String.fromCodePoint(c) : '�';
-    }
-
-    return out;
+    return decoder.decode(bytes);
 }