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/SurfaceManifestModels.cs

@@ -30,7 +30,7 @@ public sealed record SurfaceManifestDocument
 
     [JsonPropertyName("generatedAt")]
     public DateTimeOffset GeneratedAt { get; init; }
-        = DateTimeOffset.UtcNow;
+        = DateTimeOffset.MinValue;
 
     [JsonPropertyName("source")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]