perf: optimize SQL editor for large files and fix tab switching bugs

[datlechin]

Summary

• SQL editor performance: Eliminates 100% CPU and multi-second freezes when editing/executing queries in large SQL files (40MB+, 170k lines)
• Tab switching bugs: Fixes wrong query content appearing when opening table tabs or SQL files while another query tab is open
• Line highlight ghost: Fixes stale current-line highlight persisting on persisted tab load

Performance Optimizations

• Rewrite SQLContextAnalyzer to use NSString UTF-16 character-at-index ops instead of O(n²) Swift String index operations
• Add local window extraction (~10KB around cursor) in CompletionEngine for documents >500K UTF-16 units
• Early-exit in extractQueryAtCursor — stop scanning once the cursor's statement is found instead of parsing the entire file
• Batch @Published tab array mutations (isExecuting, executionTime, errorMessage) into single writes to reduce SwiftUI update cycles
• Replace O(n) Swift String ops with O(1) NSString in auto-pairing, import truncation, and export capacity reservation
• Skip persistence for queries >500KB to prevent UserDefaults/JSON encoding freeze
• Add SyntaxHighlighter reentrancy guard to prevent didProcessEditing loop
• LineNumberView draw() enumerates layout manager line fragments (O(visible lines)) instead of iterating full line cache

Bug Fixes

• Fix queryTextBinding writing to wrong tab during tab switch by using tab ID lookup instead of selectedTabIndex
• Fix SQL file open showing old tab content by desyncing version counter in handleTabSwitch to force text view sync
• Fix current-line highlight ghost on persisted tabs: clip highlight drawing to dirty rect and initialize cursor tracking after setup
• Support opening SQL files from Finder while connected

Test plan

• Open a 40MB+ SQL file — typing should be responsive with no CPU spikes
• Execute a single statement (Cmd+Enter) in a large file — no freeze before execution
• Autocomplete should work in large files (extracts local window around cursor)
• Open a table tab while a query tab is open — table data loads correctly
• Open a SQL file from Finder while a query tab is open — new tab shows file content
• Persisted tabs should show correct single-line highlight on load
• Tab switching should preserve correct query content in each tab
• Line numbers should render correctly and update on scroll in large files

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