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/functions.sh

@@ -193,81 +193,6 @@ remove_stamp() {
     rm -f "$stamp_file"
 }
 
-# Compute hash of source files in a directory
-# Usage: compute_source_hash /path/to/source "*.cpp" "*.h"
-compute_source_hash() {
-    local source_dir="$1"
-    shift
-    local patterns=("$@")
-
-    # Build find command for all patterns
-    local find_args=()
-    for pattern in "${patterns[@]}"; do
-        if [ ${#find_args[@]} -gt 0 ]; then
-            find_args+=("-o")
-        fi
-        find_args+=("-name" "$pattern")
-    done
-
-    # Hash all matching files (sorted for consistency)
-    # Use md5 on macOS, md5sum on Linux (Docker)
-    if command -v md5 &>/dev/null; then
-        find "$source_dir" -type f \( "${find_args[@]}" \) 2>/dev/null | \
-            sort | \
-            xargs cat 2>/dev/null | \
-            md5
-    else
-        find "$source_dir" -type f \( "${find_args[@]}" \) 2>/dev/null | \
-            sort | \
-            xargs cat 2>/dev/null | \
-            md5sum | \
-            cut -d' ' -f1
-    fi
-}
-
-# Create stamp with source hash
-# Usage: create_source_stamp "wxwidgets" /path/to/source "*.cpp" "*.h"
-create_source_stamp() {
-    local name="$1"
-    local source_dir="$2"
-    shift 2
-    local patterns=("$@")
-
-    local stamp_dir="${BUILD_ROOT:-$PROJECT_ROOT/build-wasm}/stamps"
-    mkdir -p "$stamp_dir"
-    local stamp_file="$stamp_dir/$name.stamp"
-
-    local hash=$(compute_source_hash "$source_dir" "${patterns[@]}")
-    echo "$hash" > "$stamp_file"
-    log_info "Created stamp: $name (hash: ${hash:0:8}...)"
-}
-
-# Check if source stamp is still valid
-# Usage: check_source_stamp "wxwidgets" /path/to/source "*.cpp" "*.h"
-# Returns: 0 if valid (no rebuild needed), 1 if invalid (rebuild needed)
-check_source_stamp() {
-    local name="$1"
-    local source_dir="$2"
-    shift 2
-    local patterns=("$@")
-
-    local stamp_file="${BUILD_ROOT:-$PROJECT_ROOT/build-wasm}/stamps/$name.stamp"
-
-    if [ ! -f "$stamp_file" ]; then
-        return 1  # No stamp, need build
-    fi
-
-    local stored_hash=$(cat "$stamp_file")
-    local current_hash=$(compute_source_hash "$source_dir" "${patterns[@]}")
-
-    if [ "$stored_hash" = "$current_hash" ]; then
-        return 0  # Up to date
-    else
-        log_info "Source changed for $name (${stored_hash:0:8}... -> ${current_hash:0:8}...)"
-        return 1  # Changed, need rebuild
-    fi
-}
-
 # Build if stamp doesn't exist
 build_if_needed() {
     local name="$1"