stigmem before 0.9.0a12 Cross-Tenant BOLA via Tombstones

stigmem-node before 0.9.0a12 contains a cross-tenant broken object level authorization (BOLA) flaw in the RTBF (right-to-be-forgotten) tombstone mechanism. issue_tombstone defaulted the tenant to "default" instead of the caller's tenant, allowing deletion records to be written to the wrong tenant, and the read-suppression path (_get_tombstone_filter and the tombstone scope cache) lacked a tenant_id predicate, so tombstone suppression was applied tenant-blind across fact queries and provenance reads. As a result, a tenant's deletion could be attributed to the wrong tenant and tombstone suppression could either hide facts belonging to other tenants or fail to hide facts within the correct tenant, undermining data isolation and RTBF guarantees. The issue is exploitable only on multi-tenant deployments running the opt-in stigmem-plugin-multi-tenant; single-tenant deployments are unaffected. Fixed in 0.9.0a12.