Rename MarkdownRenderer to Markdown

[cameronhotchkies]

Per the OOP naming convention (object→noun, method→verb), MarkdownRenderer is an -er noun — an objectified verb. Rename to Markdown with a render() method.

Scope

• core/posts/MarkdownRenderer.kt → core/posts/Markdown.kt (interface)
• adapters/markdown/SanitizingMarkdownRenderer.kt → adapters/markdown/SanitizingMarkdown.kt
• Update all references: PostService, PostsModule, SanitizingMarkdownRendererTest

Acceptance Criteria

• Interface is named Markdown, method remains render()
• All tests pass
• No behavioral change