Add call graph fixtures for various languages and scenarios

- Introduced `all-edge-reasons.json` to test edge resolution reasons in .NET.
- Added `all-visibility-levels.json` to validate method visibility levels in .NET.
- Created `dotnet-aspnetcore-minimal.json` for a minimal ASP.NET Core application.
- Included `go-gin-api.json` for a Go Gin API application structure.
- Added `java-spring-boot.json` for the Spring PetClinic application in Java.
- Introduced `legacy-no-schema.json` for legacy application structure without schema.
- Created `node-express-api.json` for an Express.js API application structure.

src/Scanner/__Libraries/StellaOps.Scanner.Analyzers.Native/Internal/Callgraph/NativeCallgraphBuilder.cs

@@ -258,6 +258,9 @@ internal sealed class NativeCallgraphBuilder
             var isResolved = targetSym.Value != 0 || targetSym.SectionIndex != 0;
             var calleePurl = isResolved ? GeneratePurl(elf.Path, targetSym.Name) : null;
 
+            // Determine edge reason based on whether target is external
+            var edgeReason = isResolved ? NativeEdgeReason.DirectCall : NativeEdgeReason.NativeInterop;
+
             _edges.Add(new NativeCallEdge(
                 EdgeId: edgeId,
                 CallerId: callerId,
@@ -265,6 +268,7 @@ internal sealed class NativeCallgraphBuilder
                 CalleePurl: calleePurl,
                 CalleeSymbolDigest: calleeDigest,
                 EdgeType: NativeEdgeType.Relocation,
+                EdgeReason: edgeReason,
                 CallSiteOffset: reloc.Offset,
                 IsResolved: isResolved,
                 Confidence: isResolved ? 1.0 : 0.5));
@@ -321,6 +325,7 @@ internal sealed class NativeCallgraphBuilder
                     CalleePurl: GeneratePurl(elf.Path, targetSym.Name),
                     CalleeSymbolDigest: targetDigest,
                     EdgeType: NativeEdgeType.InitArray,
+                    EdgeReason: NativeEdgeReason.InitCallback,
                     CallSiteOffset: (ulong)idx,
                     IsResolved: true,
                     Confidence: 1.0));

src/Scanner/__Libraries/StellaOps.Scanner.Analyzers.Native/Internal/Graph/NativeReachabilityGraph.cs

@@ -49,6 +49,7 @@ public sealed record NativeFunctionNode(
 /// <param name="CalleePurl">PURL of the callee if resolvable.</param>
 /// <param name="CalleeSymbolDigest">Symbol digest of the callee.</param>
 /// <param name="EdgeType">Type of edge (direct, plt, got, reloc).</param>
+/// <param name="EdgeReason">Semantic reason for the edge (DirectCall, NativeInterop, etc.).</param>
 /// <param name="CallSiteOffset">Offset within caller where call occurs.</param>
 /// <param name="IsResolved">Whether the callee was successfully resolved.</param>
 /// <param name="Confidence">Confidence level (1.0 for resolved, lower for heuristic).</param>
@@ -59,10 +60,30 @@ public sealed record NativeCallEdge(
     string? CalleePurl,
     string? CalleeSymbolDigest,
     NativeEdgeType EdgeType,
+    NativeEdgeReason EdgeReason,
     ulong CallSiteOffset,
     bool IsResolved,
     double Confidence);
 
+/// <summary>
+/// Semantic reason for why a native edge exists.
+/// Maps to the schema's EdgeReason enum for explainability.
+/// </summary>
+public enum NativeEdgeReason
+{
+    /// <summary>Direct function call within the same binary.</summary>
+    DirectCall,
+
+    /// <summary>Call through PLT/GOT to external library (native interop).</summary>
+    NativeInterop,
+
+    /// <summary>Initialization or finalization callback.</summary>
+    InitCallback,
+
+    /// <summary>Indirect call through function pointer (unknown target).</summary>
+    Unknown
+}
+
 /// <summary>
 /// Type of call edge.
 /// </summary>