Add curated essays and reading paths

Introduce a Library page and wire it into the main nav.
Add blog frontmatter fields: category, featuredEssay (default false),
and readingOrder (positive integer). Update several posts to mark
featured essays and assign readingOrder for the recommended path.

src/content/config.ts

@@ -7,6 +7,9 @@ const blog = defineCollection({
         description: z.string(),
         pubDate: z.coerce.date(),
         tags: z.array(z.string()).default([]),
+        category: z.string().optional(),
+        featuredEssay: z.boolean().optional().default(false),
+        readingOrder: z.number().int().positive().optional(),
         series: z
             .object({
                 name: z.string(),