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/README.md

@@ -0,0 +1,67 @@
+# vtracer (Python)
+
+Python bindings for the [`vtracer`](https://github.com/visioncortex/vtracer)
+raster-to-vector framework. Built with [pyo3](https://pyo3.rs) +
+[maturin](https://www.maturin.rs); the core Rust crate stays pure (no I/O), and
+this crate adds image decoding and a Pythonic API.
+
+## Install
+
+```sh
+pip install vtracer
+```
+
+## Usage
+
+```python
+import vtracer
+
+# one-liners
+vtracer.convert_file("in.png", "out.svg")
+svg = vtracer.convert_bytes(open("in.png", "rb").read())          # -> str
+svg = vtracer.convert_pixels(rgba_bytes, width, height)           # raw RGBA8
+
+# a rich, reusable configuration object
+cfg = vtracer.Config(mode="polygon", filter_speckle=8)
+cfg.hierarchical = "cutout"          # seam-free mosaic
+cfg.palette = ["#1b1b1b", "#e0c088", "#5a7d3c"]   # snap to a fixed palette
+cfg.max_colors = 8                   # or auto-quantize
+cfg.optimize = 2
+svg = cfg.convert_bytes(data)
+
+# presets
+vtracer.Config.poster().convert_file("photo.jpg", "poster.svg")
+vtracer.Config.bw().convert_file("scan.png", "lineart.svg")
+```
+
+### `Config`
+
+Constructor keyword arguments (all optional) — also exposed as mutable
+properties, plus the presets `Config.bw()`, `Config.poster()`, `Config.photo()`:
+
+| arg | default | notes |
+|---|---|---|
+| `color_mode` | `"color"` | `"color"` or `"bw"` |
+| `hierarchical` | `"stacked"` | `"stacked"` or `"cutout"` (mosaic) |
+| `mode` | `"spline"` | `"pixel"`, `"polygon"`, `"spline"` |
+| `filter_speckle` | `4` | discard patches smaller than X px |
+| `color_precision` | `6` | significant bits per channel |
+| `layer_difference` | `16` | color diff between gradient layers |
+| `corner_threshold` | `60` | degrees |
+| `length_threshold` | `4.0` | px |
+| `max_iterations` | `10` | |
+| `splice_threshold` | `45` | degrees |
+| `path_precision` | `2` | output decimal places |
+| `palette` | `None` | list of `#rrggbb` strings |
+| `max_colors` | `None` | auto-quantize target |
+| `optimize` | `1` | `0` off, `1` quantize+simplify, `2` + shorthands |
+
+Each `Config` has `convert_file(input, output)`, `convert_bytes(data, format=None) -> str`,
+and `convert_pixels(rgba, width, height) -> str`.
+
+## Build from source
+
+```sh
+maturin develop            # into the active virtualenv
+maturin build --release    # produce a wheel
+```