create m:n relationship from <T1> to <T2> [as <name>] generates a
junction table (compound PK over the two FK column sets, CASCADE FKs)
plus two 1:n relationships, in one do_create_table call = one undo
step. Forks user-confirmed; /runda DA pass verified the reuse against
code and the no-PK-tables-exist-in-advanced-mode fact (parent-PK guard
retained). Self-referential m:n refused; FK cols named {table}_{pkcol}.
claude@clouddev1
e598008ecf