feat(rembg): add RemoveBG Studio - native background removal app

C++ WebView2 host app driving a Python/rembg engine as a JSON-stdio
sidecar, with live before/after preview for images and video frames.
Also includes the original CLI (removebg.py) built on the same
core/ engine.

The WebView2 SDK is fetched on demand via
native/third_party/fetch-webview2.ps1 (wired into the Makefile)
rather than vendored, since it's ~15MB of prebuilt/generated
Microsoft SDK content. A Makefile provides make sync/build/run/clean
as the standard entry points.

rembg/.gitignore

@@ -0,0 +1,10 @@
+.venv/
+*.egg-info/
+
+native/build/
+native/.webview2/
+
+# Fetched by native/third_party/fetch-webview2.ps1 (or `make` /
+# `make configure`) — not vendored, too large/binary to track sensibly.
+native/third_party/webview2/include/*.h
+native/third_party/webview2/lib/