fix: allow empty description on draft essays

[ftvision]

Summary

Stops the noisy stderr spam during dev — six "description is required" errors that fired every time any essay page rendered (because getAllEssays is called from RelatedEssays, sitemap, feed, and JSON-LD, and every call parses every draft essay).

The cause

Drafts are work-in-progress. By definition the description hasn't been decided yet. But validateFrontmatter required a non-empty description for every essay, including drafts. Each parse threw, getEssayMeta caught and logged, the essay was silently dropped from results. Functionally fine, very loud.

After PR #113 (RelatedEssays) this multiplied: every essay page render triggers a fresh getAllEssays call, so the same six errors fire on every page render in dev.

The fix

validateFrontmatter (essays) and validateSeriesFrontmatter:

• If draft: true, description may be missing or empty.
• If not draft, description is still required and must be non-empty.
• Empty description normalizes to '' in the returned EssayMeta so the description: string type contract is preserved. Consumers that render description already handle empty gracefully (truncation, conditional render, OG card omits the line).

Tests

Four new cases:

• Non-draft with missing description → throws (regression guard)
• Non-draft with empty-string description → throws
• Draft with missing description → passes, normalizes to ''
• Draft with empty-string description → passes

177 total tests pass.

Remaining errors

After this fix, only one essay still complains during build:

Error parsing essay meta llm-comparison: Error: Invalid frontmatter:
  title is required and must be a string
  description is required and must be a string
  ...

llm-comparison.mdx has zero frontmatter and is not marked draft. It's also untracked (sitting in your working tree, not committed). Fix by either:

• Adding minimal frontmatter (title, date, type, topics, draft: true)
• Deleting the file (untracked, safe to remove)

Out of scope for this PR — that's a content fix, not a code fix.

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