publish(task 3): frontmatter-parser mit yaml + body-split

parseFrontmatter(md) trennt yaml-frontmatter vom markdown-body via
regex, parst yaml mit @std/yaml. Frontmatter-interface enthält
nostr-publish-konvention: optionale images-liste mit file/role/alt/
license/authors plus Author-interface (name/url/orcid). 4 tests grün.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

publish/src/core/frontmatter.ts

@@ -0,0 +1,47 @@
+import { parse as parseYaml } from '@std/yaml'
+
+export interface Author {
+  name: string
+  url?: string
+  orcid?: string
+  [key: string]: unknown
+}
+
+export interface ImageEntry {
+  file: string
+  role?: 'cover'
+  alt: string
+  caption?: string
+  license: string | 'UNKNOWN'
+  authors: Author[] | 'UNKNOWN'
+  source_url?: string
+  modifications?: string
+  [key: string]: unknown
+}
+
+export interface Frontmatter {
+  title: string
+  slug: string
+  date: Date
+  description?: string
+  image?: string
+  cover?: { image?: string; alt?: string; caption?: string }
+  tags?: string[]
+  draft?: boolean
+  license?: string
+  authors?: Author[]
+  images?: ImageEntry[]
+  [key: string]: unknown
+}
+
+export function parseFrontmatter(md: string): { fm: Frontmatter; body: string } {
+  const match = md.match(/^---\r?\n([\s\S]*?)\r?\n---\r?\n([\s\S]*)$/)
+  if (!match) {
+    throw new Error('Frontmatter: no leading --- / --- block found')
+  }
+  const fm = parseYaml(match[1]) as Frontmatter
+  if (!fm || typeof fm !== 'object') {
+    throw new Error('Frontmatter: YAML did not produce an object')
+  }
+  return { fm, body: match[2] }
+}

publish/tests/fixtures/sample-post.md

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+---
+layout: post
+title: "Sample Title"
+slug: "sample-slug"
+description: "A short summary"
+image: cover.png
+cover:
+  image: cover.png
+  alt: "Alt text"
+date: 2024-01-15
+tags: ["Foo", "Bar"]
+draft: false
+---
+
+Body content here.
+
+![pic](image1.jpg)

publish/tests/frontmatter_test.ts

@@ -0,0 +1,27 @@
+import { assertEquals, assertThrows } from '@std/assert'
+import { parseFrontmatter } from '../src/core/frontmatter.ts'
+
+Deno.test('parseFrontmatter: zerlegt Frontmatter und Body', async () => {
+  const md = await Deno.readTextFile('./tests/fixtures/sample-post.md')
+  const { fm, body } = parseFrontmatter(md)
+  assertEquals(fm.title, 'Sample Title')
+  assertEquals(fm.slug, 'sample-slug')
+  assertEquals(fm.date instanceof Date, true)
+  assertEquals(fm.tags, ['Foo', 'Bar'])
+  assertEquals(fm.cover?.image, 'cover.png')
+  assertEquals(body.trim().startsWith('Body content here.'), true)
+})
+
+Deno.test('parseFrontmatter: wirft bei fehlendem Frontmatter', () => {
+  assertThrows(() => parseFrontmatter('no frontmatter here'), Error, 'Frontmatter')
+})
+
+Deno.test('parseFrontmatter: wirft bei unvollständigem Frontmatter', () => {
+  assertThrows(() => parseFrontmatter('---\ntitle: x\n'), Error, 'Frontmatter')
+})
+
+Deno.test('parseFrontmatter: erhält Leerzeichen in String-Werten', () => {
+  const md = '---\ntitle: "Hello World"\nslug: "h-w"\ndate: 2024-01-01\n---\n\nbody'
+  const { fm } = parseFrontmatter(md)
+  assertEquals(fm.title, 'Hello World')
+})