Lay groundwork for user-editable templates by adding Tera as a
runtime template engine alongside the existing maud templates.
Changes:
- Add tera dependency
- Create TemplateEngine struct with render methods
- Add TemplateLoad/TemplateRender error variants
- Add section_type/template fields to Frontmatter
- Create templates/ directory with base, page, section, and content templates
Dead code warnings are expected; TemplateEngine will be wired
in to replace maud in subsequent commits.
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