fix(pipeline): make the full module build work for module-only projects

`crucible module build` runs the full pipeline (module + HAK producer +
topdata). A module-only project consumes prebuilt HAKs and has no
`paths.assets`, so the HAK stage crashed on the unset assets dir: the
music-credits walk hit `filepath.WalkDir("")` (`lstat : no such file or
directory`) and asset collection hit `filepath.Rel(root, "")`.

Guard both on an unset `AssetsDir()`: an absent assets tree simply means
no authored credits and no asset files to collect, so the HAK stage
no-ops cleanly while topdata generated-2DA assets still build. Honors
spec Decision 3 (canonical `crucible module build`, no `build-module`).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/pipeline/pipeline_test.go

@@ -4084,3 +4084,62 @@ haks:
 		t.Fatalf("plan-only should not stage music, stat err=%v", err)
 	}
 }
+
+// A module-only project (consumes prebuilt HAKs, no paths.assets) must build via
+// the full `crucible module build` (pipeline.Build) without an assets tree. The
+// HAK + music stages used to crash on the unset assets dir
+// (`lstat : no such file or directory` / `Rel: can't make "" relative`).
+func TestBuildModuleOnlyProjectWithoutAssetsDir(t *testing.T) {
+	root := t.TempDir()
+	mustMkdir(t, filepath.Join(root, "src", "module"))
+	mustMkdir(t, filepath.Join(root, "build"))
+
+	mustWriteFile(t, filepath.Join(root, "nwn-tool.yaml"), `
+module:
+  name: Test Module
+  resref: testmod
+paths:
+  source: src
+  build: build
+`)
+	mustWriteFile(t, filepath.Join(root, "src", "module", "module.ifo.json"), `{
+  "file_type": "IFO ",
+  "file_version": "V3.2",
+  "root": {
+    "struct_type": 0,
+    "fields": [
+      {
+        "label": "Mod_Name",
+        "type": "CExoString",
+        "value": "Test Module"
+      }
+    ]
+  }
+}
+`)
+
+	p, err := project.Load(root)
+	if err != nil {
+		t.Fatalf("load project: %v", err)
+	}
+	if err := p.ValidateLayout(); err != nil {
+		t.Fatalf("validate layout: %v", err)
+	}
+	if err := p.Scan(); err != nil {
+		t.Fatalf("scan: %v", err)
+	}
+	if p.AssetsDir() != "" {
+		t.Fatalf("test precondition: expected unset assets dir, got %q", p.AssetsDir())
+	}
+
+	result, err := Build(p)
+	if err != nil {
+		t.Fatalf("full build of module-only project: %v", err)
+	}
+	if _, err := os.Stat(result.ModulePath); err != nil {
+		t.Fatalf("expected built .mod at %s: %v", result.ModulePath, err)
+	}
+	if result.HAKAssets != 0 {
+		t.Fatalf("module-only project has no assets, expected 0 HAK assets, got %d", result.HAKAssets)
+	}
+}