fix: add missing JSDoc, return types, and as-any comments to layout engines

src/shared/lib/helpers/TextLayoutEngine/TextLayoutEngine.svelte.ts

@@ -24,7 +24,11 @@ export interface LayoutLine {
     }>;
 }
 
+/**
+ * Aggregated output of a single-font layout pass.
+ */
 export interface LayoutResult {
+    /** Per-line grapheme data. Empty when input text is empty. */
     lines: LayoutLine[];
     /** Total height in pixels. Equals `lines.length * lineHeight` (pretext guarantee). */
     totalHeight: number;
@@ -61,6 +65,7 @@ export class TextLayoutEngine {
      */
     #segmenter: Intl.Segmenter;
 
+    /** @param locale BCP 47 language tag passed to Intl.Segmenter. Defaults to the runtime locale. */
     constructor(locale?: string) {
         this.#segmenter = new Intl.Segmenter(locale, { granularity: 'grapheme' });
     }