Survey of the constraint persistence machinery revealed that
table-level/multi-column CHECK needs a NEW __rdbms_* metadata table
(SQLite exposes no PRAGMA for CHECK), unlike per-column CHECK/DEFAULT
(reuse __rdbms_playground_columns.check_expr + PRAGMA dflt_value) and
composite UNIQUE (PRAGMA index_list origin 'u' + a TableSchema field).
User-confirmed split: 4a.2 = per-column CHECK/DEFAULT (raw sql_expr
text) + composite UNIQUE(a,b), no new internal table; 4a.3 = table-level
CHECK + the new metadata table. ADR §13 and README updated in lockstep;
4a.2 plan doc added.
claude@clouddev1