feat(hint): H2 Phase B — per-form keying + the three exemplars (ADR-0053)

The first exemplar (`add 1:n relationship`) showed per-node keying is
too coarse for multi-form commands, so revise the mechanism to per-form.

- CommandNode `hint_id: Option<&str>` -> `hint_ids: &[&str]` (mirrors
  usage_ids); hint_key_for_input_in_mode reuses a factored-out
  pick_form_key (shared digit/m:n/suffix form disambiguation with
  usage_key_for_input_in_mode)
- wire INSERT + ADD (all four forms) with hint_ids
- author the three approved exemplars: hint.cmd.insert,
  hint.cmd.add_relationship, hint.err.foreign_key.child_side
  (what/example/concept) + keys.rs registration
- revise ADR-0053 D3 to per-form; record clause-concept hints as a
  deferred extension (issue #37); update README + plan
- +5 tests; 2488 pass / 1 ignored, clippy clean

src/dsl/grammar/ddl.rs

@@ -968,7 +968,7 @@ pub static DROP: CommandNode = CommandNode {
     shape: DROP_SHAPE,
     ast_builder: build_drop,
     help_id: Some("ddl.drop"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &[
         "parse.usage.drop_table",
         "parse.usage.drop_column",
@@ -982,7 +982,16 @@ pub static ADD: CommandNode = CommandNode {
     shape: ADD_SHAPE,
     ast_builder: build_add,
     help_id: Some("ddl.add"),
-    hint_id: None,
+    // Per-form (ADR-0053 D3): every form is listed so the form-word
+    // disambiguation resolves correctly; forms without an authored
+    // block yet fall back to tier-2 at render. `add_relationship` is
+    // authored as a Phase-B exemplar.
+    hint_ids: &[
+        "add_column",
+        "add_relationship",
+        "add_index",
+        "add_constraint",
+    ],
     usage_ids: &[
         "parse.usage.add_column",
         "parse.usage.add_relationship",
@@ -995,7 +1004,7 @@ pub static RENAME: CommandNode = CommandNode {
     shape: RENAME_COLUMN,
     ast_builder: build_rename_column,
     help_id: Some("ddl.rename"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.rename_column"],};
 
 pub static CHANGE: CommandNode = CommandNode {
@@ -1003,7 +1012,7 @@ pub static CHANGE: CommandNode = CommandNode {
     shape: CHANGE_COLUMN,
     ast_builder: build_change_column,
     help_id: Some("ddl.change"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.change_column"],};
 
 // =================================================================
@@ -1364,7 +1373,7 @@ pub static CREATE: CommandNode = CommandNode {
     shape: CREATE_TABLE,
     ast_builder: build_create_table,
     help_id: Some("ddl.create"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.create_table"],};
 
 // =================================================================
@@ -1433,7 +1442,7 @@ pub static CREATE_M2N: CommandNode = CommandNode {
     shape: CREATE_M2N_SHAPE,
     ast_builder: build_create_m2n,
     help_id: Some("ddl.create_m2n"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.create_m2n"],
 };
 
@@ -1864,7 +1873,7 @@ pub static SQL_CREATE_TABLE: CommandNode = CommandNode {
     shape: Node::Subgrammar(&super::sql_create_table::SQL_CREATE_TABLE_SHAPE),
     ast_builder: build_sql_create_table,
     help_id: Some("ddl.sql_create_table"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.sql_create_table"],
 };
 
@@ -1884,7 +1893,7 @@ pub static SQL_DROP_TABLE: CommandNode = CommandNode {
     shape: SQL_DROP_TABLE_SHAPE,
     ast_builder: build_sql_drop_table,
     help_id: Some("ddl.sql_drop_table"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.sql_drop_table"],
 };
 
@@ -1904,7 +1913,7 @@ pub static SQL_DROP_INDEX: CommandNode = CommandNode {
     shape: SQL_DROP_INDEX_SHAPE,
     ast_builder: build_sql_drop_index,
     help_id: Some("ddl.sql_drop_index"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.sql_drop_index"],
 };
 
@@ -1986,7 +1995,7 @@ pub static SQL_CREATE_INDEX: CommandNode = CommandNode {
     shape: SQL_CREATE_INDEX_SHAPE,
     ast_builder: build_sql_create_index,
     help_id: Some("ddl.sql_create_index"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.sql_create_index"],
 };
 
@@ -2545,7 +2554,7 @@ pub static SQL_ALTER_TABLE: CommandNode = CommandNode {
     shape: SQL_ALTER_TABLE_SHAPE,
     ast_builder: build_sql_alter_table,
     help_id: Some("ddl.sql_alter_table"),
-    hint_id: None,
+    hint_ids: &[],
     usage_ids: &["parse.usage.sql_alter_table"],
 };