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/music.go

@@ -435,33 +435,38 @@ func writeCreditsArtifacts(p *project.Project, generated []musicCreditGroup) (cr
 		}
 	}
 
-	err := filepath.WalkDir(p.AssetsDir(), func(path string, d os.DirEntry, walkErr error) error {
-		if walkErr != nil {
-			return walkErr
-		}
-		if d.IsDir() {
+	// Authored credits.md files live under the assets tree. A project with no
+	// configured assets dir (e.g. a module that consumes prebuilt HAKs) has none
+	// to collect — walking "" would fail with `lstat : no such file or directory`.
+	if assetsDir := p.AssetsDir(); assetsDir != "" {
+		err := filepath.WalkDir(assetsDir, func(path string, d os.DirEntry, walkErr error) error {
+			if walkErr != nil {
+				return walkErr
+			}
+			if d.IsDir() {
+				return nil
+			}
+			if strings.ToLower(d.Name()) != "credits.md" {
+				return nil
+			}
+			entries, err := music.ParseCreditsMarkdown(path)
+			if err != nil {
+				return fmt.Errorf("parse credits %s: %w", path, err)
+			}
+			rel, err := filepath.Rel(p.Root, path)
+			if err != nil {
+				return err
+			}
+			sources = append(sources, music.CreditsSource{
+				Path:    filepath.ToSlash(rel),
+				Kind:    "authored",
+				Entries: entries,
+			})
 			return nil
-		}
-		if strings.ToLower(d.Name()) != "credits.md" {
-			return nil
-		}
-		entries, err := music.ParseCreditsMarkdown(path)
-		if err != nil {
-			return fmt.Errorf("parse credits %s: %w", path, err)
-		}
-		rel, err := filepath.Rel(p.Root, path)
-		if err != nil {
-			return err
-		}
-		sources = append(sources, music.CreditsSource{
-			Path:    filepath.ToSlash(rel),
-			Kind:    "authored",
-			Entries: entries,
 		})
-		return nil
-	})
-	if err != nil {
-		return creditsArtifactWriteResult{}, err
+		if err != nil {
+			return creditsArtifactWriteResult{}, err
+		}
 	}
 
 	sort.Slice(sources, func(i, j int) bool { return sources[i].Path < sources[j].Path })