Materialized Views: Recovery (design + dev + review)

Crash / startup cleanup for MDEV-12163 — Materialized Views.

Scope (design + dev + code review together) — going with the Epic's 'Solution 2' (filesystem-based, no extra DDL logging):

• At startup, enumerate the data directory.
• For each file matching the materialised-view marker pattern (table_name# {version}

  ), check whether a later version of the same name exists; if so, rm_table_no_locks(file).

• Stop any in-progress materialisation: reuse the CREATE … SELECT recovery code path to discard partially-built table_name#version tables.
• Code-review iteration.

Depends on: MDEV-12163's REFRESH VIEW side ticket (defines the on-disk layout / version marker).

Notes:

• 'Solution 1' (full DDL-log-replayed deletes/renames) is kept as a Future Opportunity if Solution 2's filesystem scan ever proves insufficient.

Estimate baseline: ~6d (design + dev + code review combined).