Make opted-in abstract articles discoverable in main CirrusSearch results.

Full-text searches against the client wiki should be able to return opted-in abstract articles in the main Special:Search results page, in addition to normal mainspace results. Whether this means indexing the rendered abstract content full-text, indexing a summary only, or using a different relevance model is a decision Search Platform owns; the planning doc captures the outcome (opted-in articles are findable) rather than the mechanism.

Acceptance criteria:

• A full-text search on the client wiki that should match an opted-in abstract article returns it in the Special:Search results, visually distinguishable as an abstract-content result (if Search Platform considers visual distinction appropriate) or treated identically to mainspace results (if they do not).
• The visual-distinction decision is recorded alongside the indexing-mechanism decision in the same ADR note.
• Opted-in articles whose local title currently collides with an existing local article do not appear in results; the collision policy is inherited from M2 unchanged.
• M3's additions to the search corpus are strictly additive — ranking for non-abstract content on the client wiki is not altered by the indexing of abstract content, verified with Search Platform by running a representative set of pre-M3 and post-M3 searches against a test index and comparing the non-abstract result positions.