fix(graph): make C# namespace resolution deterministic

`buildCsNamespaceMap` previously iterated `fileSet` in fs.readdir()
traversal order, which POSIX does not guarantee. Combined with the
`candidates[0]` selection in `resolveImport`, this meant a `using`
directive could resolve to different files on different machines or
runs.

Sort the `.cs` paths lexically before scanning so candidate lists are
stable. Adds a regression test that builds the map twice from sets
populated in different orders and asserts both produce the same
candidate sequence and the same first candidate.

Found by CodeRabbit on PR #34.

src/services/graph-resolution.ts

@@ -60,12 +60,17 @@ export function buildJvmSuffixMap(fileSet: Set<string>): Map<string, string> {
  * introduced in C# 10) and builds:
  *
  *   key:   "App.Services"
- *   value: ["src/Services/UserService.cs", "src/Services/OrderService.cs"]
+ *   value: ["src/Services/OrderService.cs", "src/Services/UserService.cs"]
  *
  * Used to resolve `using App.Services;` to the candidate files that
  * contribute to that namespace. Without this, every C# `using` resolved
  * to `null` and C# projects produced an empty file-import graph.
  *
+ * Files are processed in lexicographic order so the resulting candidate
+ * lists are deterministic across machines and runs. This matters because
+ * multi-file namespaces resolve to `candidates[0]` in `resolveImport`,
+ * and a stable "first" file is required for reproducible graphs.
+ *
  * Cost: O(n) reads at graph-build time (negligible vs. AST parsing). Files
  * with no `namespace` declaration are silently skipped. Read failures are
  * swallowed since this is best-effort.
@@ -80,8 +85,13 @@ export function buildCsNamespaceMap(
   // so commented lines (`// namespace X.Y`) are not matched.
   const namespaceRegex = /^namespace\s+([\w.]+)/gm;
 
-  for (const f of fileSet) {
-    if (path.extname(f).toLowerCase() !== ".cs") continue;
+  // `fileSet` reflects fs.readdir() traversal order, which POSIX does not
+  // guarantee. Sort .cs paths lexically so candidate lists are stable.
+  const csFiles = [...fileSet]
+    .filter((f) => path.extname(f).toLowerCase() === ".cs")
+    .sort();
+
+  for (const f of csFiles) {
     let source: string;
     try {
       source = readFileSync(path.join(projectPath, f), "utf-8");