test: Add bats-core test framework

- Create test_helper/common.bash with shared utilities
- Add unit tests for firewall configuration functions
- Add unit tests for build script functions
- Establish testing patterns for TDD approach

This provides the foundation for 100% test coverage.

💘 Generated with Crush

Assisted-by: GLM-4.6 via Crush <crush@charm.land>

tests/unit/build_test.bats

@@ -0,0 +1,51 @@
+#!/usr/bin/env bats
+# Unit tests for build script functions
+
+load 'test_helper/common.bash'
+
+@test "validate_environment checks for required tools" {
+    source "${PROJECT_ROOT}/src/build-iso.sh"
+    
+    # Create mock directory structure
+    mkdir -p "${TEST_TEMP_DIR}/config"
+    mkdir -p "${TEST_TEMP_DIR}/output"
+    
+    # Override variables for testing
+    PROJECT_ROOT="$TEST_TEMP_DIR"
+    CONFIG_DIR="$TEST_TEMP_DIR/config"
+    OUTPUT_DIR="$TEST_TEMP_DIR/output"
+    
+    # Test with missing tools (should fail)
+    run validate_environment
+    assert_failure
+}
+
+@test "prepare_build creates output directory" {
+    source "${PROJECT_ROOT}/src/build-iso.sh"
+    
+    # Override variables for testing
+    PROJECT_ROOT="$TEST_TEMP_DIR"
+    OUTPUT_DIR="$TEST_TEMP_DIR/output"
+    
+    # Remove directory if it exists
+    rm -rf "$OUTPUT_DIR"
+    
+    # Run function
+    run prepare_build
+    assert_success
+    
+    # Check directory was created
+    assert [ -d "$OUTPUT_DIR" ]
+}
+
+@test "build_iso fails without live-build setup" {
+    source "${PROJECT_ROOT}/src/build-iso.sh"
+    
+    # Override variables for testing
+    PROJECT_ROOT="$TEST_TEMP_DIR"
+    OUTPUT_DIR="$TEST_TEMP_DIR/output"
+    
+    # Run function
+    run build_iso
+    assert_failure
+}