fix(diff-viewer): cache flattened search results for O(1) lookups

[matt2e]

Summary

• Cache getFlattenedResults() output using identity checks on files array and fileResults map, avoiding O(total_matches) recomputation on every call
• Build a Map<filePath, Map<localIndex, globalIndex>> lookup during flattening so getGlobalIndex() is O(1) instead of O(total_matches) linear .find()
• Reduces per-render cost from O(3 * visible * total_matches) to O(1) per visible result (after initial O(total_matches) cache build)

Context

isCurrentResult() and getGlobalIndex() are called once each per visible search result during rendering in FileSearchResults.svelte. Both called getFlattenedResults() which rebuilds the entire flattened array every time, and getGlobalIndex additionally did a .find() linear scan. With 50 visible results and 500 total matches this caused ~75,000 iterations per render.

Test plan

• Open the diff viewer and perform a search (Cmd+F or the search UI)
• Verify search results highlight correctly in the file list and diff view
• Verify "current result" indicator works when navigating with Enter/Shift+Enter
• Verify the global result counter (e.g., "3 of 47") updates correctly
• Verify clicking a search result navigates to the correct location
• Verify "Show more" / "Show less" pagination still works
• Verify search is responsive with many results across many files

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