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>

scripts/common/env.sh

@@ -34,6 +34,12 @@ export EMSDK_QUIET=1
 export EMCC_CFLAGS="-fPIC -DEMSCRIPTEN"
 export EMCC_CXXFLAGS="-fPIC -DEMSCRIPTEN -std=c++17"
 
+# Use ccache if available for faster rebuilds
+if command -v ccache &> /dev/null; then
+    export CC="ccache emcc"
+    export CXX="ccache em++"
+fi
+
 # Debug mode (default: ON, use --release to disable)
 # This can be overridden by setting DEBUG_BUILD=0 before sourcing this file
 DEBUG_BUILD="${DEBUG_BUILD:-1}"