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/friendly/strings/en-US.yaml

@@ -261,8 +261,11 @@ help:
     create: |-
         create table <T> with pk [<col>(<type>), ...]    — create a table
     sql_create_table: |-
-        create table [if not exists] <T> (<col> <type> [not null] [unique] [primary key], ...
-          [, primary key (<col>, ...)])    — create a table (advanced SQL)
+        create table [if not exists] <T> (
+          <col> <type> [not null] [unique] [primary key] [default <expr>] [check (<expr>)] [references <P>[(<col>)]], ...
+          [, primary key (<col>, ...)] [, unique (<col>, ...)] [, check (<expr>)]
+          [, [constraint <name>] foreign key (<col>) references <P>[(<col>)]])
+                                                         — create a table (advanced SQL)
     sql_drop_table: |-
         drop table [if exists] <T>                       — remove a table (advanced SQL)
     sql_create_index: |-
@@ -475,7 +478,7 @@ parse:
   # placeholders. ADR-0009's surface conventions apply.
   usage:
     create_table: "create table <Name> with pk [<col>(<type>)[, ...]]"
-    sql_create_table: "create table [if not exists] <Name> (<col> <type> [not null] [unique] [primary key], ... [, primary key (<col>, ...)])"
+    sql_create_table: "create table [if not exists] <Name> (<col> <type> [not null] [unique] [primary key] [default <expr>] [check (<expr>)] [references <Parent>[(<col>)]], ... [, primary key (<col>, ...)] [, unique (<col>, ...)] [, check (<expr>)] [, [constraint <name>] foreign key (<col>) references <Parent>[(<col>)]])"
     sql_drop_table: "drop table [if exists] <Name>"
     sql_create_index: "create [unique] index [if not exists] [<Name>] on <Table> (<col>[, ...])"
     sql_drop_index: "drop index [if exists] <Name>"