fix: correct duplicate message reporting in Go implementation

- Add DocumentSkippedError custom error type to distinguish between skipped and stored documents
- Fix counter bug where skipped messages were incorrectly reported as "stored"
- Enhance status reporting to show "X skipped as duplicates" for better visibility
- Fix Rust implementation binary attachment handling to support all file types (images, PDFs, etc.)
- Update test scripts to use correct binary names (mail2couch-go, mail2couch-rs)
- Add comprehensive test configurations for implementation comparison

Before: "Summary: Processed 30 messages, stored 30 new messages" (misleading when all were duplicates)
After: "Summary: Processed 30 messages, stored 0 new messages" with detailed "Stored 0/30 messages from INBOX (30 skipped as duplicates)"

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

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

go/couch/couch.go

@@ -15,6 +15,15 @@ import (
 	"mail2couch/mail"
 )
 
+// DocumentSkippedError indicates that a document was skipped because it already exists
+type DocumentSkippedError struct {
+	DocumentID string
+}
+
+func (e *DocumentSkippedError) Error() string {
+	return fmt.Sprintf("document %s already exists, skipped", e.DocumentID)
+}
+
 // Client wraps the Kivik client
 type Client struct {
 	*kivik.Client
@@ -149,7 +158,7 @@ func (c *Client) StoreMessage(ctx context.Context, dbName string, doc *MailDocum
 	}
 
 	if exists {
-		return nil // Document already exists, skip
+		return &DocumentSkippedError{DocumentID: doc.ID}
 	}
 
 	// Store the document first (without attachments)