Inline completion (ghost text) coordination layer

Inizia in TextViewController di CodeEditSourceEditor e leggi la primitiva di rendering dei suggerimenti inline in CodeEditTextView, quindi esamina l’API proposta di InlineCompletionDelegate e InlineCompletionItem. Esegui gli unit test esistenti con il flusso xcodebuild clean test documentato; il lavoro è completato quando richiesta, visualizzazione, accettazione, rifiuto, ciclo, controllo di Tab e cancellazione dovuta al cursore/alla modifica si comportano come descritto.

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.