feat: collision hard errors + rename resolution

- flat per-type deployed namespace enforced across own items and all sources
- own-vs-vendored and vendored-vs-vendored collisions abort with both parties named
- --rename upstream=deployed resolves collisions; on-disk folder always equals
  deployed name; rename targets are validated against new collisions
- checks run before any tree mutation; validate re-checks the namespace offline

internal/vendor/add.go

@@ -23,6 +23,7 @@ type AddOptions struct {
 	URL     string
 	Ref     string
 	Include []string
+	Renames map[string]string
 }
 
 // Add vendors a new source into the work repository at workDir: it clones,
@@ -78,6 +79,10 @@ func Add(workDir string, opts AddOptions, stdout io.Writer) error {
 		return fmt.Errorf("source %q selected zero items; refusing to add an empty source", opts.Name)
 	}
 
+	if err := checkCollisions(workDir, lf, opts.Name, selected, opts.Renames); err != nil {
+		return err
+	}
+
 	src := &lockfile.Source{
 		URL:       normalized,
 		Ref:       repo.Ref,
@@ -85,6 +90,9 @@ func Add(workDir string, opts AddOptions, stdout io.Writer) error {
 		Selection: sel,
 		Renames:   map[string]string{},
 	}
+	if len(opts.Renames) > 0 {
+		src.Renames = opts.Renames
+	}
 	if cfg != (lockfile.Discovery{}) {
 		src.Discovery = &cfg
 	}