feat: implement semantic search, knowledge unit extraction, and visualization components

src/NexusReader.Domain/Entities/KnowledgeUnitLink.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace NexusReader.Domain.Entities;
+
+public class KnowledgeUnitLink
+{
+    [Key]
+    public int Id { get; set; }
+
+    [Required]
+    [MaxLength(128)]
+    public string SourceUnitId { get; set; } = string.Empty;
+
+    [Required]
+    [MaxLength(128)]
+    public string TargetUnitId { get; set; } = string.Empty;
+
+    [Required]
+    [MaxLength(50)]
+    public string RelationType { get; set; } = "References"; // e.g., "Next", "Defines", "Contains"
+
+    [ForeignKey(nameof(SourceUnitId))]
+    public KnowledgeUnit SourceUnit { get; set; } = null!;
+
+    [ForeignKey(nameof(TargetUnitId))]
+    public KnowledgeUnit TargetUnit { get; set; } = null!;
+}