Pin InnoDB table in memory: Design

Design phase for MDEV-31492 — "Pin InnoDB table in memory".

Deliverables:

• Choose between named buffer pools vs. table-level pinning vs. low-priority-query approach
• Define DDL/SQL surface (CREATE/ALTER TABLE clause, system variables, or hints)
• Define monitoring surface (INFORMATION_SCHEMA exposure of pinned tables/pages)
• Document interaction with the rest of the buffer pool (LRU behaviour, eviction policy, sizing)
• Sign-off on the design before development starts

Estimate baseline: 25% of core development (30d) = 7d. See Julien's comment on MDEV-31492.