Add vtracer-py: Python bindings with a rich API

New crates/vtracer-py (pyo3 + maturin, abi3) wrapping the vtracer framework.
Rather than a thin CLI-style wrapper, it exposes a mutable `Config` class with
named properties and `bw`/`poster`/`photo` preset constructors, plus three
input paths — `convert_file`, `convert_bytes` (encoded image, optional format),
and `convert_pixels` (raw RGBA8) — available as `Config` methods and
module-level functions. Palette is a list of `#rrggbb` strings; bad inputs
raise ValueError.

The core crate stays pure: image decoding lives here. The crate is excluded
from the cargo workspace (pyo3 extension-module cdylibs don't link libpython,
which breaks `cargo test` at the root) and is built with maturin. Ships a
vtracer.pyi type stub. README updated.

crates/vtracer-py/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "vtracer-py"
+description = "Python bindings for the vtracer vectorization framework."
+version = "1.0.0-alpha.1"
+authors = ["Chris Tsang <tyt2y7@gmail.com>"]
+edition = "2021"
+license = "MIT OR Apache-2.0"
+homepage = "http://www.visioncortex.org/vtracer"
+repository = "https://github.com/visioncortex/vtracer/"
+
+# Excluded from the workspace: pyo3 `extension-module` cdylibs don't link
+# libpython, which breaks `cargo test` at the workspace root. Built with
+# maturin. Deps are declared explicitly (no workspace inheritance).
+
+[lib]
+# Python imports this as `vtracer`.
+name = "vtracer"
+crate-type = ["cdylib"]
+
+[dependencies]
+vtracer = { version = "1.0.0-alpha.1", path = "../vtracer" }
+image = "0.25"
+pyo3 = { version = "0.22", features = ["extension-module", "abi3-py38"] }