Add extension normalization command

internal/extension/conflicts.go

@@ -0,0 +1,58 @@
+package extension
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+type conflictDetector struct {
+	planned map[string]string
+}
+
+func newConflictDetector() *conflictDetector {
+	return &conflictDetector{planned: make(map[string]string)}
+}
+
+// evaluateTarget inspects plan collisions and filesystem conflicts. It returns true when the
+// rename can proceed, false when it must be skipped, propagating any hard errors encountered.
+func (d *conflictDetector) evaluateTarget(summary *ExtensionSummary, candidateRel, targetRel, originalAbs, targetAbs string) (bool, error) {
+	if existing, ok := d.planned[targetRel]; ok && existing != candidateRel {
+		summary.AddConflict(Conflict{
+			OriginalPath: candidateRel,
+			ProposedPath: targetRel,
+			Reason:       "duplicate_target",
+		})
+		summary.AddWarning(fmt.Sprintf("skipped %s because %s also maps to %s", candidateRel, existing, targetRel))
+		return false, nil
+	}
+
+	origInfo, origErr := os.Stat(originalAbs)
+	if origErr != nil {
+		return false, origErr
+	}
+
+	if info, err := os.Stat(targetAbs); err == nil {
+		if os.SameFile(info, origInfo) {
+			d.planned[targetRel] = candidateRel
+			return true, nil
+		}
+
+		reason := "existing_file"
+		if info.IsDir() {
+			reason = "existing_directory"
+		}
+		summary.AddConflict(Conflict{
+			OriginalPath: candidateRel,
+			ProposedPath: targetRel,
+			Reason:       reason,
+		})
+		summary.AddWarning(fmt.Sprintf("skipped %s because %s already exists", candidateRel, targetRel))
+		return false, nil
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return false, err
+	}
+
+	d.planned[targetRel] = candidateRel
+	return true, nil
+}