Fix Docker macOS timestamp issues and improve build reproducibility

- Add rsync-based source sync for Docker builds to fix macOS VirtioFS
  timestamp inconsistencies that caused autoconf sanity checks to fail
- Add config.sub wrapper (SHELL/CONFIG_SHELL) to support emscripten/wasm32
  targets without modifying submodule files
- Fix protobuf build to use native gcc/g++ for protoc instead of Emscripten
- Add python PATH fix for macOS (Homebrew's python3 symlink) in wxWidgets build

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -13,8 +13,11 @@ services:
           memory: 32G
 
     volumes:
-      # Source code (read-only - all modifications happen in build-wasm volume)
-      - ..:/workspace:ro
+      # Host source mounted read-only at staging location
+      # (rsync'd to /workspace on container start to fix macOS timestamp issues)
+      - ..:/workspace-host:ro
+      # Synced source in Docker volume (consistent timestamps)
+      - workspace-src:/workspace
       # Named volume for build cache (faster than bind mount on macOS)
       - kicad-build-cache:/workspace/build-wasm
       # Output directory for easy access to final WASM files
@@ -26,3 +29,4 @@ services:
 
 volumes:
   kicad-build-cache:
+  workspace-src: