fetchkit: refine ranking and snippet results

[gaelic-ghost]

Summary

• refine conventional-search ranking so title hits get a modest boost, SearchKit scores are normalized per field, and cross-field matches accumulate more intentionally
• replace the old single-term snippet behavior with shared query-aware snippets that can highlight multiple query terms and show truncation markers when context is cropped
• add default-path and opt-in SearchKit coverage for ranking preference, phrase behavior, and snippet presentation

Verification

• swift test
• bash scripts/repo-maintenance/run-searchkit-tests.sh
• bash scripts/repo-maintenance/validate-all.sh

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep snippet match ranges aligned with emitted text

buildSnippet computes matchRanges from raw snippetBounds, but the emitted snippetText is produced by truncatedSnippetText, which trims whitespace and can prepend/append …. Because the ranges are not adjusted after those edits, any cropped snippet can return offsets that no longer point at the matched terms in FetchSnippet.text, causing incorrect or missing highlights in consumers that trust matchRanges.

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[chatgpt-codex-connector]

Preserve exact-phrase behavior across index backends

The in-memory path now strips quotes before exact-phrase matching, so a query like "bright apple" can match there, but the SearchKit backend still wraps the raw query text in quotes (SearchKitFetchIndex.searchString(for:kind:)), which makes quoted input a literal-quote phrase query. This introduces backend-dependent behavior for the same FetchSearchQuery input and can silently drop results when callers include quotes in .exactPhrase searches.

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