Designs the remaining C3 surface: the four column-level
constraints declared in the column-spec suffix at `create
table` / `add column`, and modified on existing columns via
`add constraint … to` / `drop constraint … from`.
- A pre-flight dry-run (the ADR-0017 ethos) scans a populated
column before applying NOT NULL / UNIQUE / CHECK and refuses
with a pretty-table of offending rows; no `--force`.
- CHECK reuses the ADR-0026 expression grammar via Subgrammar.
- `__rdbms_playground_columns` carries a new `check_expr`
column; the other three are recoverable from SQLite pragmas.
- README index updated.
claude@clouddev1
7bfd213ab3