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/Cargo.toml

@@ -0,0 +1,27 @@
+[package]
+name = "vtracer-wasm"
+description = "WebAssembly core for the vtracer Node.js package."
+version = "1.0.0-alpha.1"
+authors = ["Chris Tsang <tyt2y7@gmail.com>"]
+edition = "2021"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/visioncortex/vtracer/"
+
+# Not the core workspace: this is a wasm-bindgen cdylib built with wasm-pack as
+# the Node package's native core. The Node layer does file I/O; image decoding
+# happens here in wasm, so the package has no native dependency.
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+vtracer = { version = "1.0.0-alpha.1", path = "../crates/vtracer" }
+wasm-bindgen = "0.2"
+serde = { version = "1", features = ["derive"] }
+serde-wasm-bindgen = "0.6"
+# Pure-Rust decoders that compile to wasm32-unknown-unknown.
+image = { version = "0.25", default-features = false, features = ["png", "jpeg", "gif", "bmp"] }
+
+[profile.release]
+opt-level = "s"
+lto = true