feat(scanner): Complete PoE implementation with Windows compatibility fix

- Fix namespace conflicts (Subgraph → PoESubgraph)
- Add hash sanitization for Windows filesystem (colon → underscore)
- Update all test mocks to use It.IsAny<>()
- Add direct orchestrator unit tests
- All 8 PoE tests now passing (100% success rate)
- Complete SPRINT_3500_0001_0001 documentation

Fixes compilation errors and Windows filesystem compatibility issues.
Tests: 8/8 passing
Files: 8 modified, 1 new test, 1 completion report

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/Signals/StellaOps.Signals/Storage/PoECasStore.cs

@@ -121,7 +121,9 @@ public class PoECasStore : IPoECasStore
 
         foreach (var subdir in subdirs)
         {
-            var poeHash = Path.GetFileName(subdir);
+            // Convert filesystem name back to hash format (blake3_hex -> blake3:hex)
+            var sanitizedHash = Path.GetFileName(subdir);
+            var poeHash = sanitizedHash.Replace("_", ":");
             var artifact = await FetchAsync(poeHash, cancellationToken);
 
             if (artifact != null)
@@ -153,16 +155,23 @@ public class PoECasStore : IPoECasStore
         Path.Combine(_casRoot, "reachability", "poe");
 
     private string GetPoEPath(string poeHash) =>
-        Path.Combine(GetPoeDirectory(), poeHash, "poe.json");
+        Path.Combine(GetPoeDirectory(), SanitizeHashForFilesystem(poeHash), "poe.json");
 
     private string GetDssePath(string poeHash) =>
-        Path.Combine(GetPoeDirectory(), poeHash, "poe.json.dsse");
+        Path.Combine(GetPoeDirectory(), SanitizeHashForFilesystem(poeHash), "poe.json.dsse");
 
     private string GetRekorPath(string poeHash) =>
-        Path.Combine(GetPoeDirectory(), poeHash, "poe.json.rekor");
+        Path.Combine(GetPoeDirectory(), SanitizeHashForFilesystem(poeHash), "poe.json.rekor");
 
     private string GetMetaPath(string poeHash) =>
-        Path.Combine(GetPoeDirectory(), poeHash, "poe.json.meta");
+        Path.Combine(GetPoeDirectory(), SanitizeHashForFilesystem(poeHash), "poe.json.meta");
+
+    /// <summary>
+    /// Sanitizes PoE hash for use as a filesystem directory name.
+    /// Converts "blake3:hexstring" to "blake3_hexstring" to avoid Windows colon restrictions.
+    /// </summary>
+    private static string SanitizeHashForFilesystem(string poeHash) =>
+        poeHash.Replace(":", "_");
 
     private string ComputeHash(byte[] data)
     {