Default relevance weighting for FTS search (title vs. body)

[marcusbellamyshaw-cell]

Problem

FTSManager.enableSearch() calls bm25() with no weights argument unless the
caller explicitly passes one, so every searchable field counts equally toward
ranking. A result whose title matches the query ranks the same as a result that
merely mentions the query term once, incidentally, somewhere in a long body — both
are "correct" matches, but users expect the title-relevant one to rank higher.

weights is already supported end-to-end (setSearchConfig, POST /_emdash/api/search/enable) — sites can fix this today by passing explicit
per-field weights. The ask here is a better default, since most sites won't
discover or configure weights on their own.

Suggestion

Default to weighting the title field higher than other searchable fields when no
explicit weights are supplied — e.g. title: 3, everything else: 1 — rather than
the current implicit all-1 default. Open to whatever ratio the maintainers think is
right; flagging the problem and a starting number, not insisting on this exact one.

Alternatives considered

• Leave the default as-is and only improve docs pointing at weights. Doesn't help
  sites that never read that doc section.
• Require weights explicitly (no default) — breaking change, not worth it for a
  ranking quality-of-life improvement.