vnassets/background: add bg removal via rembg + isnet-anime

rembg with ONNX Runtime on CPU was chosen over BiRefNet: the latter
requires deform_conv2d which crashes on ROCm bfloat16 and runs at 40s
in float32. rembg delivers 0.23s per 1024x1024 image, no GPU deps,
and isnet-anime is trained specifically on anime images — exactly the
target domain.

CLI and session API both support single-image and batch (plural-first)
modes, reusing one model session across files.

README.md

@@ -139,6 +139,43 @@ Either model can be omitted (`None`) for single-model sessions. Properties:
 The standalone `vnasset generate` and `vnasset edit` CLI commands are thin
 wrappers around a one-shot session — same API, backwards compatible.
 
+### Background Removal
+
+Remove backgrounds from character sprites (output is RGBA PNG):
+
+```bash
+# Single file
+vnasset remove-bg --input character_base.png --output character_transparent.png
+
+# Batch (reuses model across files)
+vnasset remove-bg --input base.png --input happy.png --input sad.png --output-dir transparent/
+```
+
+Or via the session API:
+
+```python
+with VnAssetsSession() as vna:
+    vna.remove_background("base.png", output="base_transparent.png")
+
+    # Batch
+    vna.remove_backgrounds(
+        ["base.png", "happy.png", "sad.png"],
+        output_dir="transparent/",
+    )
+```
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `--input` | (required, repeatable) | Input image path(s) |
+| `--output` | (auto) | Output path (single mode) |
+| `--output-dir` | (none) | Output directory (batch mode) |
+| `--model` | `isnet-anime` | Model: `isnet-anime`, `u2net`, `u2netp`, `u2net_human_seg`, `isnet-general-use`, `sam` |
+
+The default `isnet-anime` model is trained on anime images — ideal for the
+novaAnimeXL art style. Inference runs on CPU via onnxruntime (~0.25s per
+1024×1024 image on Strix Halo). The model is ~176 MB, downloaded on first use
+to `~/.u2net/`.
+
 ## Architecture
 
 ```
@@ -350,7 +387,8 @@ Use `--raw` to bypass weighting and fall back to plain diffusers encoding.
 | Compel prompt weighting + BREAK | ✅ Working |
 | Lightning LoRA fuse-at-load | ✅ Working |
 | Flash attention (experimental) | ✅ Working |
-| Metadata JSON output | ✅ Working |
+| `vnasset remove-bg` | ✅ Working |
+| Session background removal | ✅ Working |
 | `vnasset pipeline` (batch YAML config) | 🚧 Planned |
 | `vnasset serve` (daemon/HTTP API) | 🚧 Planned |
 | `torch.compile` on UNet | 🚧 Planned |
@@ -371,5 +409,3 @@ Use `--raw` to bypass weighting and fall back to plain diffusers encoding.
   availability or a ComfyUI installation.
 - **`vnasset serve`** — lightweight daemon with Unix socket or HTTP API for
   integrating VNAsset into external tools.
-- **Background removal** — green-screen trim and transparency pass for
-  post-processing sprites.