Add configurable custom 404 page generation (GitHub Pages compatible)

[Copilot]

Adds opt-in generation of a static 404.html at the site root, compatible with GitHub Pages custom 404 page conventions. Default behavior is unchanged — the page is only generated when Generate404Page: true is set in site configuration.

Changes

Core model & enum

• SiteSettings.Generate404Page — new bool property, defaults to false
• TemplateType.NotFound — new enum value to identify 404 output files

New generator (PPTail.Generator.NotFound)

• INotFoundProvider interface with Generate404Page() method
• NotFoundProvider implementation generates self-contained HTML from site settings (Title, Description, Copyright) with proper HTML encoding — no template required per design

Site builder

• Builder now validates INotFoundProvider is registered and conditionally emits ./404.html when siteSettings.Generate404Page is true

Configuration

• YamlSiteSettings (MediaBlog) and Forestry.SiteSettings updated to deserialize and map the new property

Enabling the feature

In your SiteSettings.md YAML front matter:

Generate404Page: true

Tests

• 2 constructor dependency tests for INotFoundProvider
• 6 build tests covering enabled/disabled behavior, output filename, content passthrough, and provider invocation count