fix: implement cross-registry layer copying for layered images

Resolves GitHub issue #28 by implementing proper layered image building
that preserves base image properties and handles cross-registry scenarios.

Changes:
- Enhanced push_layered_image() to detect cross-registry pushes and copy base layers
- Implemented automatic blob copying from source registry to destination registry
- Always use layered approach instead of falling back to single-layer builds
- Added verification tests to ensure base image environment and files are preserved
- Consolidated unit tests into module files for better organization

This fixes "MANIFEST_BLOB_UNKNOWN" errors when pushing layered images that
reference base image blobs from different registries (e.g., cgr.dev → ttl.sh).

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

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

example/hello-krust/src/main.rs

@@ -1,4 +1,17 @@
 fn main() {
     println!("Hello from krust example!");
-    println!("This is running inside a container built with krust.");
+    println!("Current architecture: {}", std::env::consts::ARCH);
+
+    // Check for SSL_CERT_FILE environment variable (common in distroless images)
+    match std::env::var("SSL_CERT_FILE") {
+        Ok(value) => println!("✓ SSL_CERT_FILE found: {}", value),
+        Err(_) => panic!("✗ SSL_CERT_FILE not found (base image env not preserved)"),
+    }
+
+    // Check for /etc/os-release file (common in most Linux base images)
+    if std::path::Path::new("/etc/os-release").exists() {
+        println!("✓ /etc/os-release found (base image layers preserved)");
+    } else {
+        panic!("✗ /etc/os-release not found (base image layers not preserved)");
+    }
 }