Simple Memory v3.10.0

What's changed

• Allow explicit memory relationships across spaces.
• Require write access to both endpoint spaces when creating or removing cross-space links.
• Restrict traversal and relationship-expanded search to spaces the caller can read.
• Hide links through soft-deleted or inaccessible spaces and restore them with the space.
• Add migration v9 with automatic legacy-link backfill and pre-migration backup.
• Add focused cross-space migration, authorization, pagination, search-expansion, and lifecycle coverage.