fix(app): deduplicate diff summaries linearly

[Hona]

Summary

• replace quadratic timeline diff-summary deduplication with a Set-backed reverse scan
• preserve legacy filtering, ordering, last-duplicate selection, and object identity

Fixes #33106.

Verification

• black-box differential check: 21,845 input sequences matched legacy output order and identity
• focused unit tests pass
• app typecheck passes
• app build passes
• 11,878 unique-diff benchmark: 494.9 ms to 1.2 ms
• pre-push workspace typecheck passes

Full app unit run: 622 passed; the existing Arabic i18n parity failure remains outside the changed files.

[Copilot]

Pull request overview

This PR addresses a performance bottleneck in the app’s session timeline construction by replacing the prior quadratic diff-summary deduplication with a Set-backed linear reverse scan, matching the legacy behavior while avoiding renderer hangs on large summary.diffs payloads.

Changes:

• Introduces uniqueSummaryDiffs() to deduplicate SnapshotFileDiff entries in linear time while preserving legacy ordering and “last diff wins” semantics.
• Updates timeline row construction to use the new helper instead of inline reduceRight(... some(...)) logic.
• Adds unit tests verifying filtering, ordering, and object identity preservation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/app/src/pages/session/timeline/summary-diffs.ts	Adds Set-backed linear deduplication helper for summary diffs with legacy-compatible ordering.
packages/app/src/pages/session/timeline/summary-diffs.test.ts	Adds coverage for invalid entries, last-duplicate selection, display order, and identity preservation.
packages/app/src/pages/session/timeline/rows.ts	Replaces inline quadratic dedup logic with uniqueSummaryDiffs() and removes the local type guard.

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