fix: prevent syntax highlighting from disappearing after initial display

[batonogov]

Summary

Fixes #556 — syntax highlighting shows correctly when a file is first opened, then disappears after a short time.

Root cause: A cascading re-highlight cycle during makeNSView:

1. textView.string = text fired textDidChange (delegate was already set before text assignment)
2. textDidChange wrote parent.text → updateContent → cachedHighlightResult = nil, contentVersion++
3. Synchronous highlight applied colors successfully (colors visible briefly)
4. SwiftUI re-render → updateNSView → updateContentIfNeeded saw textChanged = true (from contentVersion bump) AND languageChanged = true (lastLanguage was "")
5. textView.string = text (same text!) stripped all highlight attributes since isRichText = false
6. Re-highlight attempted but could be delayed or overwritten by further cycles

Fixes applied:

• Defer delegate assignment in makeNSView until after text and highlighting are configured, preventing textDidChange from firing during initial setup
• Initialize lastLanguage/lastFileName from parent in Coordinator.init, eliminating false languageChanged on first updateNSView
• Guard textView.string = text with textView.string != text to avoid stripping attributes when contentVersion bumps but text content is identical
• Skip cached highlight result when language changed (stale grammar matches from old language)

Test plan

• New HighlightPersistenceTests (8 tests) covering:
  • Coordinator initializes lastLanguage/lastFileName from parent
  • First updateNSView skips unnecessary update when language matches
  • Same-content version bump preserves highlight colors
  • Different content correctly replaces text
  • Language change triggers fresh highlight (not stale cache)
  • Delegate set after text does not fire textDidChange
  • Delegate after text preserves highlight attributes
  • cancelPendingHighlight increments generation
• Updated CodeEditorStaticTests.coordinatorInitialState for new init behavior
• All existing coordinator, highlight cache, async highlight, and concurrent highlight tests pass
• SwiftLint clean on all changed files

[github-actions]

✅ Code Coverage: 73.3%

Threshold: 70%

Coverage is above the minimum threshold.

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