Inline completion (ghost text) coordination layer

CodeEditSourceEditor の TextViewController から始め、CodeEditTextView にあるインライン候補の描画プリミティブを確認し、その後、提案されている InlineCompletionDelegate および InlineCompletionItem API をレビューします。文書化されている xcodebuild clean test のフローで既存のユニットテストを実行します。リクエスト、表示、受け入れ、破棄、サイクル、Tab の制御、カーソル移動・編集によるクリアが、説明どおりに動作すれば完了です。

Description

Add an inline completion (ghost text) coordination layer to CodeEditSourceEditor, built on the inline suggestion rendering primitive in CodeEditTextView. This gives the editor a provider-agnostic way to request, show, accept, cycle, and dismiss inline AI suggestions (the GitHub Copilot UI pattern).

Proposed API:

• InlineCompletionDelegate: a @MainActor protocol a host conforms to. inlineCompletionsRequested(textView:cursorPosition:) async -> [InlineCompletionItem] returns suggestions; optional inlineCompletionDidShow/Accept/Dismiss hooks report lifecycle.
• InlineCompletionItem: the suggestion model (insert text plus the range it applies to).
• TextViewController methods: requestInlineSuggestion() (debounced), setInlineSuggestions(_:selectedIndex:), acceptInlineSuggestion(), dismissInlineSuggestion(), selectNext/PreviousInlineSuggestion().
• Behavior: requests are debounced and skipped while the completion popup is visible (ghost text and popup never co-exist); Tab accepts when a suggestion is active; moving the cursor or editing dismisses the ghost text.

Alternatives Considered

• Putting this logic in the app target (CodeEdit) directly. Rejected because the request/accept/dismiss lifecycle is tightly coupled to the editor's cursor, selection, and key handling, which live here. Keeping it in CodeEditSourceEditor lets any host provide completions through a small delegate, rather than each host re-implementing the lifecycle.
• Mutating the document to preview suggestions. Rejected; the underlying CodeEditTextView primitive renders ghost text as an overlay without touching text storage, so previews never pollute undo or layout.

Additional Context

This is the middle layer of a planned native inline-completion feature for the CodeEdit stack: CodeEditTextView provides the rendering primitive (CodeEditApp/CodeEditTextView#124), CodeEditSourceEditor (this issue) coordinates the lifecycle, and CodeEdit wires up a concrete provider such as GitHub Copilot.

I have a working, documented implementation with unit tests covering request/show, accept, dismiss, cycling, Tab-accept gating, and ghost-text clearing on cursor move and edit. It builds and tests pass via xcodebuild ... clean test (against the CodeEditTextView change above), and SwiftLint is clean. Would the maintainers welcome this in CodeEditSourceEditor?

Screenshots

The user-visible ghost text is shown by the app-level integration in CodeEdit; this layer is the coordination API and is covered by unit tests.