Matrix: pin natural candidate ordering

8 tests covering completion-candidate order: connective keywords in
reading order (`to`/`from`/`in` before `table`), and command-part
keywords before schema identifiers. The ordering already held via
declaration-order preservation + keywords-first sectioning in
candidates_at_cursor; nothing pinned it until now, so a future
grammar or sort change could silently break the hint panel's
left-to-right reading.

tests/typing_surface/mod.rs

@@ -34,6 +34,7 @@ pub mod drop_relationship;
 pub mod add_relationship;
 pub mod rename_change_column;
 pub mod app_commands;
+pub mod candidate_ordering;
 
 // =========================================================
 // Canonical schema shapes (handoff §1 — CANONICAL_SCHEMA_SHAPES)