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/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 	"time"
@@ -174,16 +175,26 @@ func processImapSource(source *config.MailSource, couchClient *couch.Client, dbN
 		stored := 0
 		if !dryRun {
 			ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+			var skipped int
 			for i, doc := range docs {
 				err := couchClient.StoreMessage(ctx, dbName, doc, messages[i])
 				if err != nil {
-					log.Printf("     ERROR: Failed to store message %s: %v", doc.ID, err)
+					var skipErr *couch.DocumentSkippedError
+					if errors.As(err, &skipErr) {
+						skipped++
+					} else {
+						log.Printf("     ERROR: Failed to store message %s: %v", doc.ID, err)
+					}
 				} else {
 					stored++
 				}
 			}
 			cancel()
-			fmt.Printf("     Stored %d/%d messages from %s\n", stored, len(messages), mailbox)
+			if skipped > 0 {
+				fmt.Printf("     Stored %d/%d messages from %s (%d skipped as duplicates)\n", stored, len(messages), mailbox, skipped)
+			} else {
+				fmt.Printf("     Stored %d/%d messages from %s\n", stored, len(messages), mailbox)
+			}
 		} else {
 			stored = len(messages) // In dry-run, assume all would be stored
 			fmt.Printf("     DRY-RUN: Would store %d messages from %s\n", len(messages), mailbox)