GHSA-x26h-xmv8-gxf7: stigmem-node: RTBF tombstones are mis-attributed and suppress reads tenant-blind (cross-tenant BOLA)

On a multi-tenant stigmem node, RTBF (right-to-be-forgotten) tombstones were mis-scoped two ways. (1) issue_tombstone defaulted the tenant to "default" instead of the caller’s tenant, so tombstones could be written to the wrong tenant. (2) The read-suppression path — _get_tombstone_filter (routes/facts/common.py) and the _tombstone_scope_cache (lifecycle/tombstones.py) — had no tenant_id predicate, so tombstone suppression was applied tenant-blind across fact queries and provenance. Reached via /v1/tombstones and the fact query/provenance read paths.