feat: ADR-0035 4i(a,b) — CREATE TABLE help/usage + describe table constraints; Phase 4 complete

(b) describe shows table-level constraints: TableDescription gains
unique_constraints + check_constraints (populated by do_describe_table
from read_schema), rendered in a new "Table constraints:" section —
composite UNIQUE and table-level CHECK (named + unnamed). The per-column
Constraints column already covered single-column NOT NULL/UNIQUE/PK/CHECK.

(a) CREATE TABLE help/usage skeleton refreshed for the column DEFAULT/
CHECK/REFERENCES, table-level composite UNIQUE, table CHECK, and
table-level FOREIGN KEY forms (4a.2/4a.3/4b) — engine-neutral,
vocab-audit clean.

With 4i's (c)/(d)/(e) already shipped, this completes sub-phase 4i — the
verification sweep — and therefore ADR-0035 Phase 4 (4a–4i). ADR-0035
Status, §13 4i, the ADR index, and requirements.md Q1 updated to
"Phase 4 complete".

Tests: render_structure table-level-constraints unit test +
e2e_describe_shows_table_level_constraints. Full suite 1917 passing /
0 failing / 1 ignored; clippy clean.

src/db.rs

@@ -70,6 +70,13 @@ pub struct TableDescription {
     pub inbound_relationships: Vec<RelationshipEnd>,
     /// User-created indexes on this table (ADR-0025).
     pub indexes: Vec<IndexInfo>,
+    /// Table-level composite `UNIQUE (a, b, …)` constraints (ADR-0035
+    /// §4a.2). Single-column UNIQUE rides on the column itself
+    /// (`ColumnDescription::unique`); these are the multi-column ones.
+    pub unique_constraints: Vec<Vec<String>>,
+    /// Table-level `CHECK (…)` constraints with their optional name
+    /// (ADR-0035 §4a.3 / §4g), in declaration order.
+    pub check_constraints: Vec<crate::persistence::TableCheck>,
 }
 
 /// One user-created index on a table (ADR-0025).
@@ -7390,6 +7397,8 @@ fn do_describe_table(conn: &Connection, name: &str) -> Result<TableDescription,
         outbound_relationships,
         inbound_relationships,
         indexes,
+        unique_constraints: schema.unique_constraints.clone(),
+        check_constraints: schema.check_constraints,
     })
 }