perf: Add uv cache to Docker build

The uv cache contains all Python wheel files (807MB) and is the primary
cache that speeds up PDM installations. This was previously missing from
the cache warming strategy.

Changes:
- Added /opt/chipflow-cache/uv directory
- Copy ~/.cache/uv to /opt/chipflow-cache/uv at end of Docker build
- Copy /opt/chipflow-cache/uv to ~/.cache/uv in post-create.sh

Cache sizes in Docker image:
- PDM: 116KB (metadata only)
- uv: 807MB (Python wheels - main cache!)
- yowasp-yosys: 126MB (WASM JIT compilation)
- zig: 79MB (Zig toolchain compilation)

Total: ~1.01GB cached in prebuild

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: robtaylor

.devcontainer/Dockerfile

@@ -50,7 +50,7 @@ RUN --mount=type=bind,target=/src \
     cp /src/pyproject.toml . && \
     cp /src/pdm.lock . && \
     pdm config use_uv true && \
-    mkdir -p /opt/chipflow-cache/pdm /opt/chipflow-cache/yowasp /opt/chipflow-cache/zig && \
+    mkdir -p /opt/chipflow-cache/pdm /opt/chipflow-cache/yowasp /opt/chipflow-cache/zig /opt/chipflow-cache/uv && \
     pdm config cache_dir /opt/chipflow-cache/pdm && \
     export YOWASP_CACHE_DIR=/opt/chipflow-cache/yowasp && \
     export ZIG_GLOBAL_CACHE_DIR=/opt/chipflow-cache/zig && \
@@ -67,4 +67,6 @@ RUN --mount=type=bind,target=/src \
     pdm run chipflow pin lock && \
     pdm run chipflow sim build && \
     cd .. && \
-    rm -rf cache-warming-temp
+    rm -rf cache-warming-temp && \
+    echo "Copying uv cache to /opt/chipflow-cache" && \
+    cp -r ~/.cache/uv/* /opt/chipflow-cache/uv/

.devcontainer/post-create.sh

@@ -20,6 +20,16 @@ if [ -n "$CODESPACE_NAME" ]; then
     RETRY_DELAY=3
     HTTP_CODE="404"
 
+    # Copy uv cache from Docker image (contains Python wheels - 800MB+)
+    echo "🔥 Copying uv cache..."
+    mkdir -p ~/.cache/uv
+    if [ -d /opt/chipflow-cache/uv ] && [ "$(ls -A /opt/chipflow-cache/uv)" ]; then
+        cp -r /opt/chipflow-cache/uv/* ~/.cache/uv/
+        echo "✅ uv cache copied"
+    else
+        echo "⚠️  No uv cache found"
+    fi
+
     # Copy PDM cache from Docker image
     echo "🔥 Copying PDM cache..."
     mkdir -p ~/.cache/pdm