perf: reduce memory usage by ~80-130 MB per connection

[datlechin]

Summary

Implements 8 memory/CPU optimizations identified in the memory audit (see docs/development/MEMORY-AUDIT.md):

• Eliminate dedicated ping driver (~30–50 MB per connection): Health checks now use the main driver instead of maintaining a separate C-level driver instance
• Eliminate metadata driver (~30–50 MB per connection): Metadata queries (columns, FKs, row count) multiplex on the main driver via C-level DispatchQueue serialization. No second TCP connection needed.
• Lazy-load plugin bundles (~20–30 MB at launch): Plugin discovery (Info.plist) happens at startup, but bundle.load() is deferred until the first driver request or settings screen visit
• Cross-window tab eviction (reclaim ~90 MB per evicted tab): Inactive native window-tabs evict row data after 5s via NSWindow.didResignKeyNotification, re-fetched on focus
• Remove RowBuffer.sourceQuery (<500 KB per tab): Eliminated duplicate query string; rehydration reads from tab.query directly
• Zero-copy InMemoryRowProvider (~3–10 MB per tab): Provider now references RowBuffer directly instead of copying rows; sorted views store index permutation instead of reordered row arrays
• Window-scoped NSWindow.didUpdateNotification (CPU): Each editor's notification handler now early-returns for notifications from other windows — only 1 handler does work per update cycle instead of N

Also clears InMemoryRowProvider cache when RowBuffer is evicted, fixing a bug where eviction freed RowBuffer.rows but the provider still held its own copy.

Files changed

• ConnectionSession.swift — removed metadataDriver property
• DatabaseManager.swift — removed ping driver, metadata driver, ensureMetadataDriver, metadataCreationTasks; health checks use main driver
• PluginManager.swift — split loadAllPlugins into discoverPlugins + loadPendingPlugins
• DatabaseDriver.swift — trigger loadPendingPlugins() on first driver creation
• RowProvider.swift — InMemoryRowProvider holds RowBuffer reference + optional sortIndices
• MainEditorContentView.swift — makeRowProvider() passes rowBuffer + sort indices
• MainContentCoordinator.swift — Phase 2 metadata uses main driver; evictInactiveRowData() method
• MainContentCoordinator+Navigation.swift — removed all 8 metadata driver sync blocks
• SQLEditorCoordinator.swift — filter notifications by editor's own window
• QueryTab.swift — removed RowBuffer.sourceQuery
• ExportDialog.swift, InstalledPluginsView.swift — trigger lazy plugin load on appear

Test plan

• Connect to MySQL — verify connection works and health monitoring pings succeed
• Open table tab — verify columns, FKs, and column defaults load correctly (Phase 2 on main driver)
• Open 3+ table tabs as native window-tabs — switch away, wait >5s, switch back — verify data reloads
• Switch database — verify tables reload (no metadata driver to keep in sync)
• Open Settings > Plugins — verify all plugins appear
• Export dialog — verify all export formats available
• Verify sorted query results display correctly
• Profile with Instruments Allocations to measure actual savings vs baseline

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