Materialized Views: Testing & debugging

QA, integration testing, and debug iteration for MDEV-12163 — Materialized Views.

Scope:

• Functional: CREATE / REFRESH / DROP / SHOW CREATE VIEW, privilege handling (per the Epic: users have access rights to the view).
• Versioning visibility: long-transaction read-stability under concurrent REFRESH (new transactions see latest; the long one stays pinned on its version).
• Concurrency: deny concurrent REFRESH; correctness under LOCK TABLES / UNLOCK TABLES.
• Crash recovery: kill mid-materialisation, restart, verify cleanup; kill mid-REFRESH, verify the in-progress version is removed and the previous version remains valid.
• Storage-engine sanity: Aria as the underlying storage — verify behaviour under non-transactional storage semantics (no half-states observable).
• Edge cases: DROP MATERIALIZED VIEW while a long-running transaction is reading it; storage-quota variable behaviour at the limit; views referencing each other / nested.
• Bug fix iteration: this ticket absorbs the QA-found bug fixes across the REFRESH / DML / Recovery dev tickets.

Depends on: the three sibling dev tickets (REFRESH VIEW side, DML, Recovery).

Estimate baseline: 40% of core development (25d) = 10d.