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/build.go

@@ -661,11 +661,17 @@ func collectAssetResources(p *project.Project, requireContent bool, allowed map[
 	if err != nil {
 		return nil, err
 	}
-	assetsRelPath, err := filepath.Rel(p.Root, p.AssetsDir())
-	if err != nil {
-		return nil, fmt.Errorf("resolve assets dir relative path: %w", err)
+	// A module-only project (consumes prebuilt HAKs, has no paths.assets) has no
+	// asset files to collect — AssetFiles is empty and AssetsDir() is "". Skip the
+	// relative-path resolve, which would otherwise fail with `Rel: can't make ""`.
+	assetsRelPath := ""
+	if p.AssetsDir() != "" {
+		assetsRelPath, err = filepath.Rel(p.Root, p.AssetsDir())
+		if err != nil {
+			return nil, fmt.Errorf("resolve assets dir relative path: %w", err)
+		}
+		assetsRelPath = filepath.ToSlash(assetsRelPath)
 	}
-	assetsRelPath = filepath.ToSlash(assetsRelPath)
 
 	for _, rel := range p.Inventory.AssetFiles {
 		if musicAssets != nil {