docs(avlivebody-mac): contributor setup README

Document prerequisites, mlpackage copy, signing xcconfig, and
xcodegen/xcodebuild commands. Points at design spec and plan.

avlivebody-mac/README.md

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+# AVLiveBody (macOS)
+
+Native macOS Xcode app that renders SMPL-X body meshes in RealityKit
+from the USB iPhone body-tracking pipeline (ARBodyTracker -> Multi-HMR
+worker -> AVLiveBody scene).
+
+## Prerequisites
+
+- macOS 15+
+- Xcode 16+
+- `xcodegen` — `brew install xcodegen`
+
+## First-time setup
+
+1. Copy the CoreML model into the app resources (required, gitignored,
+   ~195 MB). Without it the app degrades to skeleton-only rendering:
+
+   ```
+   cp -R ~/.cache/av-live-multihmr/multihmr_full_672_s.mlpackage \
+       avlivebody-mac/Sources/AVLiveBody/Resources/
+   ```
+
+2. Create your local xcconfig and set your signing team:
+
+   ```
+   cp Config/Local.xcconfig.example Config/Local.xcconfig
+   # Edit Config/Local.xcconfig:
+   #   DEVELOPMENT_TEAM = <your Apple Developer Team ID>
+   ```
+
+## Build
+
+Generate the Xcode project (run after every `project.yml` change) then
+open or build from the CLI:
+
+```
+cd avlivebody-mac
+xcodegen generate
+open AVLiveBody.xcodeproj
+```
+
+CLI build / test:
+
+```
+xcodebuild -project AVLiveBody.xcodeproj -scheme AVLiveBody \
+    -destination 'platform=macOS' build
+
+xcodebuild -project AVLiveBody.xcodeproj -scheme AVLiveBody \
+    -destination 'platform=macOS' test
+```
+
+## Runtime requirements
+
+A tethered iPhone running the matching `ARBodyTracker` iOS app over USB
+is required for body input. See `iphone-arbody/` for the iOS side.
+
+## Architecture
+
+- Design spec:
+  `docs/superpowers/specs/2026-05-18-avlivebody-macos-rewrite-design.md`
+- Implementation plan:
+  `docs/superpowers/plans/2026-05-18-avlivebody-macos-rewrite.md`