Implement Advisory Canonicalization and Backfill Migration

- Added AdvisoryCanonicalizer for canonicalizing advisory identifiers.
- Created EnsureAdvisoryCanonicalKeyBackfillMigration to populate advisory_key and links in advisory_raw documents.
- Introduced FileSurfaceManifestStore for managing surface manifests with file system backing.
- Developed ISurfaceManifestReader and ISurfaceManifestWriter interfaces for reading and writing manifests.
- Implemented SurfaceManifestPathBuilder for constructing paths and URIs for surface manifests.
- Added tests for FileSurfaceManifestStore to ensure correct functionality and deterministic behavior.
- Updated documentation for new features and migration steps.

src/Scanner/__Libraries/StellaOps.Scanner.Surface.FS/ISurfaceManifestReader.cs

@@ -0,0 +1,15 @@
+namespace StellaOps.Scanner.Surface.FS;
+
+/// <summary>
+/// Provides read access to published surface manifests.
+/// </summary>
+public interface ISurfaceManifestReader
+{
+    Task<SurfaceManifestDocument?> TryGetByDigestAsync(
+        string manifestDigest,
+        CancellationToken cancellationToken = default);
+
+    Task<SurfaceManifestDocument?> TryGetByUriAsync(
+        string manifestUri,
+        CancellationToken cancellationToken = default);
+}