feat(cli): add --dry-run to convert, merge and split

Read and validate inputs, apply filtering/merging in memory, validate the
output target (format, --package, --schema, --extra-fields) and print the
plan to stdout without writing any output. Closes #35.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

cmd/relspec/split.go

@@ -24,6 +24,7 @@ var (
 	splitExcludeTables  string
 	splitNullableTypes  string
 	splitNullableArrays string
+	splitDryRun         bool
 )
 
 var splitCmd = &cobra.Command{
@@ -115,6 +116,8 @@ func init() {
 	splitCmd.Flags().StringVar(&splitNullableTypes, "types", "", "Nullable type package for code-gen writers (bun/gorm): 'baselib' (default, Go pointer types), 'stdlib' (database/sql), or 'sqltypes'")
 	splitCmd.Flags().StringVar(&splitNullableArrays, "array-nullable", "", "Nullable PostgreSQL array representation for the Bun writer in stdlib/baselib --types mode: 'slice' (default, plain slice) or 'pointer_slice' (*[]T, distinguishes NULL from '{}')")
 
+	splitCmd.Flags().BoolVar(&splitDryRun, "dry-run", false, "Read, filter and validate the selection and print the plan without writing any output")
+
 	err := splitCmd.MarkFlagRequired("from")
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Error marking from flag as required: %v\n", err)
@@ -174,6 +177,26 @@ func runSplit(cmd *cobra.Command, args []string) error {
 	}
 	fmt.Fprintf(os.Stderr, "      ✓ Filtered to: %d schema(s), %d table(s)\n\n", len(filteredDB.Schemas), filteredTables)
 
+	if splitDryRun {
+		if err := validateWriteTarget(filteredDB, splitTargetType, splitPackageName, "", ""); err != nil {
+			return fmt.Errorf("dry run validation failed: %w", err)
+		}
+		out := outWriter(cmd)
+		fmt.Fprintf(out, "RelSpec split plan (dry run - nothing written):\n")
+		fmt.Fprintf(out, "  Input:  %s database '%s'\n", splitSourceType, db.Name)
+		fmt.Fprintf(out, "  Output: %s -> %s\n", splitTargetType, splitTargetPath)
+		fmt.Fprintf(out, "  Selection: %s\n", splitSelection{
+			Schemas:        parseCommaSeparated(splitSchemas),
+			Tables:         parseCommaSeparated(splitTables),
+			ExcludeSchemas: parseCommaSeparated(splitExcludeSchema),
+			ExcludeTables:  parseCommaSeparated(splitExcludeTables),
+			DatabaseName:   splitDatabaseName,
+		}.summary())
+		printDryRunPlan(out, filteredDB)
+		fmt.Fprintf(os.Stderr, "=== Dry run complete: no output written ===\n\n")
+		return nil
+	}
+
 	// Write to target format
 	fmt.Fprintf(os.Stderr, "[3/3] Writing to target format...\n")
 	fmt.Fprintf(os.Stderr, "      Format: %s\n", splitTargetType)