feat(assets): seven asset commands + Run dispatch/helpers

check-dupes, clean-dupes, check-mdl, fix-mdl, convert, upscale, compile
behind a single runner indirection, mirroring internal/depot's Run shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/assets/helpers.go

@@ -0,0 +1,84 @@
+// Package assets is the crucible `assets` builder: NWN:EE model/texture tools
+// that operate in place on a target directory. Mirrors internal/depot's
+// Run(args, stdout, stderr, getenv) shape and bypasses the legacy internal/app
+// surface entirely.
+package assets
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// runner is the single external-command indirection so tests can observe and
+// stub every engine / ImageMagick / upscaler invocation. dir is the working
+// directory ("" = inherit); env replaces the child environment when non-nil.
+var runner = func(dir string, env []string, name string, args ...string) ([]byte, error) {
+	cmd := exec.Command(name, args...)
+	cmd.Dir = dir
+	if env != nil {
+		cmd.Env = env
+	}
+	return cmd.CombinedOutput()
+}
+
+// walk collects files under each root whose lower-cased extension is in exts. A
+// root that is itself a matching file is included. recursive controls descent
+// into subdirectories. Results are sorted and deduplicated.
+func walk(roots []string, exts map[string]bool, recursive bool) ([]string, error) {
+	seen := map[string]bool{}
+	var out []string
+	add := func(p string) {
+		if exts[strings.ToLower(filepath.Ext(p))] && !seen[p] {
+			seen[p] = true
+			out = append(out, p)
+		}
+	}
+	for _, root := range roots {
+		fi, err := os.Stat(root)
+		if err != nil {
+			return nil, err
+		}
+		if !fi.IsDir() {
+			add(root)
+			continue
+		}
+		err = filepath.WalkDir(root, func(p string, d os.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			if d.IsDir() {
+				if !recursive && p != root {
+					return filepath.SkipDir
+				}
+				return nil
+			}
+			add(p)
+			return nil
+		})
+		if err != nil {
+			return nil, err
+		}
+	}
+	sort.Strings(out)
+	return out, nil
+}
+
+// look returns the first bare name found on PATH, or the first candidate that
+// is an existing path. Returns "" if none resolve.
+func look(candidates ...string) string {
+	for _, c := range candidates {
+		if strings.ContainsRune(c, filepath.Separator) {
+			if fi, err := os.Stat(c); err == nil && !fi.IsDir() {
+				return c
+			}
+			continue
+		}
+		if p, err := exec.LookPath(c); err == nil {
+			return p
+		}
+	}
+	return ""
+}