perf: tab restore takes 10+ seconds — native window creation is sequential

[datlechin]

Problem

Restoring tabs on app launch freezes the app. With 12 tabs: ~10 seconds. With 30 tabs: 24+ seconds, 100% CPU, 800MB RAM.

Root cause

Each tab = one NSWindow (macOS native tab requirement). Each NSWindow creation via SwiftUI's openWindow takes ~800ms:

• Full SwiftUI view hierarchy creation
• SessionStateFactory + MainContentCoordinator instantiation
• SourceEditor + TreeSitterClient initialization
• Schema loading race condition (12+ parallel loads)
• databaseDidConnect notification storm (24+ events)

Profiling data

[RESTORE] Disk load: 35ms, 30 tab(s)       ← Fast
[RESTORE] Opened 29 native tabs in 11541ms  ← BOTTLENECK
[RESTORE] Connected in 3163ms
[RESTORE] loadSchemaIfNeeded × 25           ← Redundant
[RESTORE] refreshTables × 12                ← Redundant

Fix direction

Option A: Internal tab bar (recommended)

Build a SwiftUI tab bar inside a single window. All tabs live in one NSWindow, only the active tab's editor is rendered. Native macOS tabs become optional (drag-out). This is how TablePlus, DataGrip, DBeaver, and VS Code work.

Option B: Lazy NSWindow creation

Keep native tabs but create lightweight placeholder NSWindows (AppKit, not SwiftUI) that only show a title. Swap in the full SwiftUI content when the user activates a tab.

Incremental improvements (already identified)

• Schema load coalescing: add Task-based deduplication on SQLSchemaProvider
• refreshTables gate: only first coordinator per connection calls refreshTables
• These reduce redundant work but don't fix the core window creation bottleneck