fix: append article subtopic content

[julianschiavo]

• Append child topic content onto parent article results in search and entry lookups.
• Discover child topics through VectorSearch identifier filters instead of SQLite or payload inspection.
• Cover the regression with live integration tests for expanded content and subtopic deduplication.

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Note

Medium Risk
Changes vector-search result shaping by fetching and merging descendant topic content and filtering out subtopic hits, which can affect search/entry output and performance. Also adds a new Objective-C selector bridge (stringIdentifiersApplyingFilters) that could fail at runtime if the private API differs across OS versions.

Overview
Search and exact entry lookups now expand article content with descendant topic content (identifiers using the parent#child convention), so parent articles include subtopic text when omitContent is false.

This adds a new VSKClientObject.stringIdentifiers(applying:) bridge to fetch identifiers by attribute filters, uses it to discover topic identifiers, fetches descendant assets in bulk, appends their content onto the parent hit, and deduplicates search results so subtopics aren’t returned separately when the parent article is present.

Adds live integration tests covering content expansion and subtopic deduplication, plus new live-environment fixtures for an article with subtopics.

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