Add nodejs: WebAssembly Node package with no native dependency

New nodejs/ package: a wasm-bindgen crate (vtracer-wasm) built with wasm-pack
that wraps the vtracer framework, plus a thin JS layer for file I/O. Image
decoding (png/jpeg/gif/bmp via the image crate) runs in wasm too, so the
package has zero native dependencies — no sharp, no node-gyp.

No separate general-purpose wasm crate: the Node package directly owns and
wraps the wasm. Excluded from the cargo workspace (wasm-bindgen cdylib), built
with wasm-pack.

JS API (camelCase options): convertBuffer, convertPixels, convertFile,
convertFileSync — each taking an Options object (preset, colorMode,
hierarchical/cutout mosaic, mode, palette, maxColors, optimize, ...). Ships
index.d.ts types and a node smoke test. README updated.

Verified: builds to wasm32-unknown-unknown; `node test.js` passes; output
matches the CLI/Python bindings (253 paths on the tank sample).

nodejs/index.d.ts

@@ -0,0 +1,34 @@
+/** Conversion options. Any field may be omitted; omitted fields use the framework default. */
+export interface Options {
+  /** Applied before other fields: "bw" | "poster" | "photo". */
+  preset?: 'bw' | 'poster' | 'photo';
+  colorMode?: 'color' | 'bw';
+  hierarchical?: 'stacked' | 'cutout';
+  mode?: 'pixel' | 'polygon' | 'spline';
+  filterSpeckle?: number;
+  colorPrecision?: number;
+  layerDifference?: number;
+  cornerThreshold?: number;
+  lengthThreshold?: number;
+  maxIterations?: number;
+  spliceThreshold?: number;
+  pathPrecision?: number;
+  /** Fixed palette: `#rrggbb` strings. */
+  palette?: string[];
+  /** Auto-quantize target color count. */
+  maxColors?: number;
+  /** 0 = off, 1 = quantize+simplify, 2 = + shorthands/grouping. */
+  optimize?: number;
+}
+
+/** Vectorize an encoded image (PNG/JPEG/GIF/BMP) buffer to an SVG string. */
+export function convertBuffer(buffer: Uint8Array, options?: Options): string;
+
+/** Vectorize a raw RGBA8 buffer (`width * height * 4` bytes) to an SVG string. */
+export function convertPixels(rgba: Uint8Array, width: number, height: number, options?: Options): string;
+
+/** Read an image file, vectorize it, and write the SVG to disk. */
+export function convertFile(inputPath: string, outputPath: string, options?: Options): Promise<void>;
+
+/** Synchronous {@link convertFile}. */
+export function convertFileSync(inputPath: string, outputPath: string, options?: Options): void;