Add ccache and incremental build system for faster development

- Add ccache to Docker image for compiled object caching
- Change build defaults: incremental by default, skip deps by default
- Add new flags: --full, --clean-kicad, --build-deps
- Skip wxWidgets configure if already configured (check Makefile timestamps)
- Remove unused source hash stamp functions (make handles dependencies)
- Update build.md with new build system documentation

Performance improvements:
- Single file change: 7:35 → 1:34 (4.8x faster)
- No-change rebuild: 7:35 → 1:31 (5x faster)
- Asyncify post-processing (~1 min) is now the bottleneck

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

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

docker/Dockerfile

@@ -17,8 +17,17 @@ RUN apt-get update && apt-get install -y \
     protobuf-compiler \
     swig \
     unixodbc-dev \
+    ccache \
     && rm -rf /var/lib/apt/lists/*
 
+# Configure ccache for Emscripten
+# CCACHE_DIR in named volume persists across containers
+ENV CCACHE_DIR=/workspace/build-wasm/.ccache
+ENV CCACHE_MAXSIZE=10G
+ENV CCACHE_COMPILERCHECK=content
+ENV CCACHE_SLOPPINESS=include_file_mtime,time_macros,pch_defines
+ENV CCACHE_BASEDIR=/workspace
+
 WORKDIR /workspace
 
 # Entry point that sources Emscripten environment