perf: lower async sort threshold to 1K rows, add display cache eviction

[datlechin]

Summary

• Async sort threshold lowered from 10K to 1K rows — sorting 1K-10K rows was happening synchronously in the SwiftUI view body, blocking the main thread for 50-100ms. Now dispatched to background via Task.detached, matching the existing async sort path.
• Display cache eviction — InMemoryRowProvider.displayCache was unbounded. For 10K rows x 50 columns, it could hold 500K entries (~5MB). Added eviction at 20K entries, keeping the half closest to the current access point (same strategy as DatabaseRowProvider).

Both thresholds are aligned: coordinator (MainContentCoordinator.swift:1173) and view (MainEditorContentView.swift:558).

Test plan

• Sort a query result with 500 rows — should sort synchronously (below threshold), instant
• Sort a query result with 2000 rows — should show brief executing state, then sorted result
• Sort a query result with 10K+ rows — should sort async as before, no regression
• Scroll through a large table (5K+ rows) — verify display cache eviction doesn't cause visible stutter (re-formatting is lazy)
• Verify no memory growth when scrolling back and forth through large results