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/run.go

@@ -0,0 +1,55 @@
+package assets
+
+import (
+	"fmt"
+	"io"
+)
+
+const (
+	exitOK    = 0
+	exitFail  = 1  // problems found / per-file failures
+	exitUsage = 64 // bad invocation / unknown subcommand / bad flags
+	exitTool  = 70 // missing external tool / internal error
+)
+
+// Run executes an assets subcommand. args[0] is the subcommand; returns the
+// process exit code.
+func Run(args []string, stdout, stderr io.Writer, getenv func(string) string) int {
+	if len(args) == 0 {
+		printUsage(stderr)
+		return exitUsage
+	}
+	rest := args[1:]
+	switch args[0] {
+	case "check-dupes":
+		return runCheckDupes(rest, stdout, stderr)
+	case "clean-dupes":
+		return runCleanDupes(rest, stdout, stderr)
+	case "check-mdl":
+		return runCheckMDL(rest, stdout, stderr)
+	case "fix-mdl":
+		return runFixMDL(rest, stdout, stderr)
+	case "convert":
+		return runConvert(rest, stdout, stderr)
+	case "upscale":
+		return runUpscale(rest, stdout, stderr)
+	case "compile":
+		return runCompile(rest, stdout, stderr, getenv)
+	default:
+		fmt.Fprintf(stderr, "assets: unknown subcommand %q\n\n", args[0])
+		printUsage(stderr)
+		return exitUsage
+	}
+}
+
+func printUsage(w io.Writer) {
+	fmt.Fprint(w, `usage:
+  assets compile      [--nwn INSTALL] [--non-recursive] <dir>...
+  assets convert      [--to dds|png|tga] [--backend PATH] [--non-recursive] <dir>...
+  assets upscale      [--scale N] [--backend PATH] [--non-recursive] <dir>...
+  assets check-mdl    <path>...
+  assets fix-mdl      [--dry-run] <path>...
+  assets check-dupes  <dir>...
+  assets clean-dupes  [--dry-run] <primary> <clean>
+`)
+}