✨ Line Annotations

### Description

Implement the visual display system for Line Annotations in the source editor. These annotations should appear directly beneath relevant lines of code and visually communicate messages such as errors, warnings, or informational notes.

### Requirements

- Add support for displaying annotations below lines in the source editor
- An annotation should include:
- A severity icon (error, warning, info)
- A short descriptive message
- Multiple annotations should be stackable if they occur on the same line
- Styling should reflect severity (e.g., color, icon)
- Annotations should update cleanly when the underlying data model changes (insert, remove, update)
- Annotations should avoid disrupting layout:
- Automatically shrink if needed to preserve line spacing.
- If the message is truncated, allow users to click the annotation to reveal a popover with the full message.
- This popover should scroll with the editor and remain positioned relative to its line.
- If the line scrolls out of view and the annotation is expanded, the popover should remain visible—docked to the top or bottom edge of the editor—until dismissed.

### Implementation Details

- The expanded popover view (when multiple annotations exist or a message is truncated) may be called AnnotationAggregatedDropdownView, similar to what is used in other macOS editors.
- The source editor (`CodeEditSourceEditor`) will accept a new property:
```swift
annotations: Set
```
- LineAnnotation is a lightweight data model that represents a message to display beneath a specific line:
```swift
struct LineAnnotation: Hashable, Identifiable {
let id: UUID = UUID()
let line: Int
let column: Int
let severity: Severity
let text: String
}

enum Severity {
case error, warning, info, live
}
```
- The editor should group annotations by line internally and render them accordingly.

### Additional Context

This is the foundational work for visual feedback in the editor. Future issues will introduce the system for feeding issues into the annotations (e.g., from builds, linters, or language servers) via an issue manager.

This feature is a key part of improving the editing experience by surfacing issues contextually, and sets the stage for deeper diagnostic and development tooling integration.

### Related Issues

- https://github.com/CodeEditApp/CodeEdit/issues/354

### Screenshots

Warning

Warning Expanded

Error

Multiple Errors

Multiple Errors Expanded

從 CodeEditSourceEditor 進入點開始，檢視提議的 LineAnnotation 模型與 AnnotationAggregatedDropdownView 概念。將需求與相關 issue #354 進行比較，然後圍繞分組且依嚴重性設定樣式的註解、乾淨的模型更新，以及考量捲動的展開與截斷行為來定義完成標準。

由索引模型根據 Issue 內容生成。