Add attribute governance design: controlled attribute lists, egress regex selection, ingest signature validation

Defines the SA-V3-Ext/Int placeholder attribute scheme (external list
owned by the S3 handover spec, internal list for SA routing + internal
S3 usage), the per-egress Regex property for packing/signing selection,
the inverse-regex delete step at true external egress, optional
ingest-time signing with mandatory validation when already signed, and
the S3 JSON-fallback size attribute for suffix-range trailer reads.

.claude/memory/packaging-signing-decisions.md

@@ -37,3 +37,9 @@ reversed an earlier direction during design review:
 - **S3-vs-XFS storage backend split by file size is unresolved** — open
   question whether S3 can serve small files efficiently enough to drop
   the XFS tier entirely.
+- **Attribute list is a controlled allowlist, not "all FlowFile
+  attributes."** ~15 controlled external attributes carry through
+  ingest-to-delivery (these are what get signed/packed); a separate
+  controlled internal set is SA-only and stripped before export. Import
+  drops uncontrolled external attributes; export drops internal ones. See
+  [[attribute-governance]].