Materialized Views: DML (design + dev + review)

Read-path / transaction integration for MDEV-12163 — Materialized Views.

Scope (design + dev + code review together):

• THD::LOMV (list of materialised views used by current transaction) data structure + lifecycle.
• Table open path: in TDC, detect 'this is a materialised view'; check LOMV; either reuse the already-pinned TABLE_SHARE (so the transaction keeps reading a consistent version) or open the latest version and add a record / bump refcount in the registry.
• Same path under LOCK TABLES (open once at lock time; refcount adjusted at UNLOCK TABLES).
• Transaction end: walk LOMV, unlink each, decrement refcount; if it drops to 0 and a newer version exists, drop the now-obsolete versioned table and remove its registry record.
• Visibility guarantee: a long-running transaction continues reading the version it pinned, while new transactions see the latest version immediately.
• Code-review iteration.

Depends on: MDEV-12163's REFRESH VIEW side ticket (storage layout + registry).

Estimate baseline: ~11d (design + dev + code review combined).