fix(fk): inline FK referencing a compound PK points at the table-level form

ADR-0043 D4 residual: an inline column-level FK (`<col> REFERENCES P(a,b)`)
is single-column by construction, so referencing a parent's compound PK
gave the generic arity error ("1 foreign-key column(s) on the child side,
but `P`'s key has 2..."). It now points the user at the table-level form:
"an inline column reference can only name one column ... Use the table-level
form instead: FOREIGN KEY (<columns>) REFERENCES P (a, b)".

- Adds `inline: bool` to SqlForeignKey, set by the grammar's single shared
  builder consume_fk_reference (true for the inline path, false for the
  table-level and ALTER paths).
- resolve_fk_parent_columns takes `inline` and tailors the arity-mismatch
  message when an inline FK meets a compound key.

Tests: parse-layer (inline=true / table-level=false) + end-to-end worker
refusal wording. 2209 pass / 0 fail / 1 ignored. Clippy clean.

tests/it/sql_drop_table.rs

@@ -109,6 +109,7 @@ fn dropping_a_referenced_parent_is_refused() {
             parent_columns: Some(vec!["id".to_string()]),
             on_delete: rdbms_playground::dsl::ReferentialAction::NoAction,
             on_update: rdbms_playground::dsl::ReferentialAction::NoAction,
+            inline: true,
         }],
         false,
         Some("create table child (id serial primary key, pid int references parent(id))".to_string()),