feat: implement just-based build system with distinct binary names

Replace Makefile with justfile for better build management:

Build System Changes:
- Add comprehensive justfile with recipes for both implementations
- Configure Go to build as `mail2couch-go`
- Configure Rust to build as `mail2couch-rs` via Cargo.toml
- Add universal build, test, clean, and install recipes
- Include development convenience recipes and utility commands

New Justfile Features:
- `just build` - builds both implementations
- `just install` - installs both to /usr/local/bin
- `just test` - runs tests for both implementations
- `just sizes` - shows binary size comparison
- `just versions` - compares version outputs
- `just --list` - shows all available recipes

Documentation Updates:
- Update CLAUDE.md with justfile usage examples
- Replace make commands with just equivalents
- Add new utility commands (sizes, versions)
- Update IMPLEMENTATION_COMPARISON.md deployment sections

Benefits of just over make:
- Better command-line interface with `--list`
- More intuitive recipe syntax
- Better suited for development workflows
- Cross-platform compatibility
- Built-in help and documentation

Binary Naming:
- Go implementation: `mail2couch-go` (11M)
- Rust implementation: `mail2couch-rs` (8.3M)
- Clear distinction prevents conflicts
- Parallel installation support

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

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

justfile

@@ -0,0 +1,113 @@
+# Justfile for mail2couch project
+# Builds both Go and Rust implementations with distinct binary names
+
+# Default recipe
+default: build
+
+# Build both implementations
+build: build-go build-rust
+
+# Build Go implementation as mail2couch-go
+build-go:
+    @echo "Building Go implementation..."
+    cd go && go build -o mail2couch-go .
+    @echo "✅ Built: go/mail2couch-go"
+
+# Build Rust implementation as mail2couch-rs  
+build-rust:
+    @echo "Building Rust implementation..."
+    cd rust && cargo build --release
+    @echo "✅ Built: rust/target/release/mail2couch-rs"
+
+# Build optimized release versions
+build-release: build-go-release build-rust-release
+
+# Build optimized Go release
+build-go-release:
+    @echo "Building optimized Go release..."
+    cd go && go build -ldflags="-s -w" -o mail2couch-go .
+    @echo "✅ Built optimized: go/mail2couch-go"
+
+# Build optimized Rust release (already built with --release above)
+build-rust-release: build-rust
+
+# Install binaries to /usr/local/bin (requires sudo)
+install: build-release
+    @echo "Installing binaries to /usr/local/bin..."
+    sudo cp go/mail2couch-go /usr/local/bin/
+    sudo cp rust/target/release/mail2couch-rs /usr/local/bin/
+    sudo chmod +x /usr/local/bin/mail2couch-go
+    sudo chmod +x /usr/local/bin/mail2couch-rs
+    @echo "✅ Installed mail2couch-go and mail2couch-rs"
+
+# Run tests for both implementations
+test: test-go test-rust
+
+# Test Go implementation
+test-go:
+    @echo "Running Go tests..."
+    cd go && go test ./...
+
+# Test Rust implementation  
+test-rust:
+    @echo "Running Rust tests..."
+    cd rust && cargo test
+
+# Clean build artifacts
+clean:
+    @echo "Cleaning build artifacts..."
+    cd go && rm -f mail2couch-go mail2couch
+    cd rust && cargo clean
+    @echo "✅ Cleaned build artifacts"
+
+# Check code formatting and linting
+check: check-go check-rust
+
+# Check Go code
+check-go:
+    @echo "Checking Go code..."
+    cd go && go fmt ./...
+    cd go && go vet ./...
+
+# Check Rust code
+check-rust:
+    @echo "Checking Rust code..."
+    cd rust && cargo fmt --check
+    cd rust && cargo clippy -- -D warnings
+
+# Fix code formatting
+fmt: fmt-go fmt-rust
+
+# Format Go code
+fmt-go:
+    @echo "Formatting Go code..."
+    cd go && go fmt ./...
+
+# Format Rust code
+fmt-rust:
+    @echo "Formatting Rust code..."
+    cd rust && cargo fmt
+
+# Development convenience - build and test Go version
+dev-go: build-go
+    ./go/mail2couch-go --help
+
+# Development convenience - build and test Rust version
+dev-rust: build-rust
+    ./rust/target/release/mail2couch-rs --help
+
+# Show binary sizes
+sizes: build
+    @echo "Binary sizes:"
+    @ls -lh go/mail2couch-go rust/target/release/mail2couch-rs | awk '{print $5 "\t" $9}'
+
+# Run both binaries with --version to compare
+versions: build
+    @echo "Go version:"
+    @./go/mail2couch-go --help | head -1
+    @echo ""
+    @echo "Rust version:"
+    @./rust/target/release/mail2couch-rs --version
+
+# Clean and rebuild everything
+rebuild: clean build