Add imaging/document primitives: PNG codec, rasterizer, GIF, PDF

- Pure-JS PNG decode (grey/RGB/palette/alpha) + RGBA encode, CRC-checked;
  decode verified byte-identical with ImageMagick
- Software rasterizer: shapes, arrows, blur, highlight, magnify, tooltip,
  number badges, cursor, bitmap text (vendored public-domain font8x8),
  crop/resize, focused-view rendering
- GIF89a encoder with LZW (cross-validated pixel-for-pixel against
  ImageMagick decode) + NETSCAPE looping
- Minimal PDF 1.4 writer: pages, fonts, rects, images, outlines, valid
  xref; rendering validated under Ghostscript
- 12 imaging workflow tests (35 total)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/gifdecode.js

@@ -0,0 +1,102 @@
+'use strict';
+
+/**
+ * Minimal GIF decoder used only by tests to verify the encoder end-to-end:
+ * parses header/palette/frames and decompresses LZW back to RGB pixels.
+ */
+
+function decodeGif(buf) {
+  if (buf.toString('latin1', 0, 6) !== 'GIF89a') throw new Error('not GIF89a');
+  const width = buf.readUInt16LE(6);
+  const height = buf.readUInt16LE(8);
+  const packed = buf[10];
+  const gctSize = packed & 0x80 ? 2 << (packed & 0x07) : 0;
+  let pos = 13;
+  const palette = buf.subarray(pos, pos + gctSize * 3);
+  pos += gctSize * 3;
+
+  const frames = [];
+  let loops = null;
+  while (pos < buf.length) {
+    const block = buf[pos++];
+    if (block === 0x3b) break; // trailer
+    if (block === 0x21) { // extension
+      const label = buf[pos++];
+      if (label === 0xff) {
+        const size = buf[pos];
+        const app = buf.toString('latin1', pos + 1, pos + 1 + size);
+        pos += 1 + size;
+        const sub = [];
+        while (buf[pos] !== 0) { sub.push(buf.subarray(pos + 1, pos + 1 + buf[pos])); pos += 1 + buf[pos]; }
+        pos++;
+        if (app.startsWith('NETSCAPE')) loops = Buffer.concat(sub).readUInt16LE(1);
+      } else {
+        while (buf[pos] !== 0) pos += 1 + buf[pos];
+        pos++;
+      }
+    } else if (block === 0x2c) { // image descriptor
+      const fw = buf.readUInt16LE(pos + 4);
+      const fh = buf.readUInt16LE(pos + 6);
+      const lpacked = buf[pos + 8];
+      pos += 9;
+      if (lpacked & 0x80) pos += (2 << (lpacked & 0x07)) * 3;
+      const minCode = buf[pos++];
+      const chunks = [];
+      while (buf[pos] !== 0) { chunks.push(buf.subarray(pos + 1, pos + 1 + buf[pos])); pos += 1 + buf[pos]; }
+      pos++;
+      const indices = lzwDecode(Buffer.concat(chunks), minCode, fw * fh);
+      frames.push({ width: fw, height: fh, indices });
+    } else {
+      throw new Error(`unknown block 0x${block.toString(16)} at ${pos - 1}`);
+    }
+  }
+  return { width, height, palette, frames, loops };
+}
+
+function lzwDecode(data, minCode, expectedPixels) {
+  const CLEAR = 1 << minCode;
+  const EOI = CLEAR + 1;
+  let codeSize = minCode + 1;
+  let dict = [];
+  const resetDict = () => {
+    dict = [];
+    for (let i = 0; i < CLEAR; i++) dict[i] = [i];
+    dict[CLEAR] = null; dict[EOI] = null;
+    codeSize = minCode + 1;
+  };
+  resetDict();
+
+  const out = [];
+  let bitPos = 0;
+  let prev = null;
+  const readCode = () => {
+    let code = 0;
+    for (let i = 0; i < codeSize; i++) {
+      const byte = data[bitPos >> 3];
+      if (byte === undefined) return -1;
+      code |= ((byte >> (bitPos & 7)) & 1) << i;
+      bitPos++;
+    }
+    return code;
+  };
+
+  for (;;) {
+    const code = readCode();
+    if (code < 0 || code === EOI) break;
+    if (code === CLEAR) { resetDict(); prev = null; continue; }
+    let entry;
+    if (dict[code]) entry = dict[code];
+    else if (code === dict.length && prev) entry = [...prev, prev[0]];
+    else throw new Error(`bad LZW code ${code} (dict ${dict.length})`);
+    out.push(...entry);
+    if (prev) {
+      dict.push([...prev, entry[0]]);
+      if (dict.length === (1 << codeSize) && codeSize < 12) codeSize++;
+    }
+    prev = entry;
+    if (out.length >= expectedPixels) break;
+  }
+  return out;
+}
+
+module.exports = { decodeGif };