perf(web): speed up session event virtualization

[Yevanchen]

Summary

• Replace the virtualized session-event drawer's per-scroll linear offset scan with binary search.
• Add boundary and mixed-row-height equivalence coverage against the previous lookup behavior.

Why

• The drawer enables virtualization above 200 events, but locating the visible range still scanned every row on each scroll update. Binary search reduces that hot lookup from O(n) to O(log n) without changing row-edge or overscan behavior.

Verification

• Commands: just check; just test-file apps/web/tests/session-event-drawer-virtualization.test.ts; just test-package @mosoo/web; just tc-package @mosoo/web; just commit-check
• Manual steps: Compared 5,000 lookups across 10,000 rows; linear lookup took 20.709 ms and binary lookup took 1.671 ms (12.4x faster), with matching checksums.
• Not run: Browser QA; the change is a pure lookup helper covered across 1,000 mixed-height rows and thousands of scroll positions.

Impact

• User/API/contract changes: No API or contract changes; large session-event drawers perform less work while scrolling.
• Generated files / GraphQL / DB / lockfile: None.
• Env or config changes: None.
• Risk and rollback: Low. The search relies on the strictly increasing offsets produced by the existing drawer layout; revert this commit to restore the prior linear scan.

Review

• Closest review areas: Exact row-boundary selection, overscan preservation, and empty-list behavior.
• Known trade-offs: The implementation intentionally preserves the previous inclusive bottom-edge semantics rather than redefining which row owns an exact boundary.

Closes YEF-1063