tests: add fixture corpus quality coverage

[gaelic-ghost]

Summary

• Add a tiny attributed Project Gutenberg fixture corpus for FetchKit result-quality tests.
• Preserve title-only snippets while exposing matchedFields and snippetField on FetchSearchResult so consumers can distinguish title evidence from body evidence.
• Cover the same fixture corpus expectations through the macOS SearchKit-backed index path.
• Record the future opt-in extended snippet/background summary direction in the roadmap.

Verification

• swift test --filter SearchKitFetchIndexTests
• swift test
• scripts/repo-maintenance/validate-all.sh
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve backwards-decoding for FetchSearchResult

Adding matchedFields as a non-optional stored property on this public Codable type causes synthesized decoding to require the key, so JSON produced before this change will now fail with keyNotFound when decoded. This impacts any caller that persists or caches FetchSearchResult across app/package upgrades; a manual init(from:) should decode matchedFields with a fallback (for example decodeIfPresent(... ) ?? []) to keep older payloads readable.

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