[bug] wiki_refine_draft writes an empty lead, title, sections or frontmatter over the draft #585

What happened

ADR-1068 refuses a wiki_refine_draft call when title, lead, sections and frontmatter are all absent. The test is is None, so an empty value passes it and is written:

• lead: "" replaces the draft's lead with an empty string. draft_curator._lead_is_placeholder then reads the draft as a placeholder and subtracts 0.25 from its score, so a call that refined nothing degrades the draft.
• frontmatter: {} replaces the draft's frontmatter, dropping the updated stamp the synthesizer wrote (core/draft_synthesizer.py).
• title: " " writes a blank title; evaluate_draft scores an empty title at −0.3.
• sections: [] reaches the kind-contract validation, which refuses it for any kind with a required section. A kind with no required section would have its sections replaced by an empty list.

Each of those also writes a refined_llm memo saying a model refined the draft.

Measured by the review of PR #584 (#584 (comment)), which drove the three shapes against a real SQLite draft: lead: "" and frontmatter: {} returned updated: true and wrote a memo.

Expected behavior

A refinement carries a value. A field present in the call but empty (an empty or whitespace-only string, an empty list, an empty object) is refused by name, and the call writes neither the draft nor a memo. A call where every content field is absent stays refused as ADR-1068 says.

Blanking a field is not a refinement; nothing in Path B asks for it. wiki_write and wiki_purge are how a page's content is replaced or removed.

Evidence required to close

Tests driving lead: "", title: " ", sections: [], frontmatter: {}, and a mixed call (a real lead plus an empty title), each asserting the draft row and the memo count are unchanged and the response names the offending field.