Make opted-in abstract articles discoverable in article title typeahead search.

When a reader types into the client wiki's search box, the typeahead suggestions (served by CirrusSearch's prefix search / title search backend) should include any opted-in abstract articles whose local title matches the reader's query, with the same typeahead UX as normal mainspace results. How this is plumbed into CirrusSearch — an auxiliary index, a custom suggester, synthetic page rows, something else — is a decision owned by Search Platform after reviewing what M2 shipped. This sub-bullet captures the outcome we want, not the mechanism.

Acceptance criteria:

• Typing the local title of an opted-in abstract article into the search box on the client wiki returns the article as a typeahead suggestion, with the same visual treatment as a normal mainspace suggestion.
• The indexing / suggester mechanism chosen with Search Platform is recorded as an ADR-style note linked from this task, naming the alternatives considered and the rationale for the choice.
• Opt-in set changes (additions, removals, title edits) are reflected in typeahead within the same cadence CirrusSearch otherwise operates at for normal page creations; no bespoke faster-invalidation path.
• Opted-in articles whose local title currently collides with an existing local article do not appear in typeahead; the local article wins in search exactly as it wins in rendering (see M2 collision policy).