ADR-0016 + Iter 5/6 follow-up: pretty table rendering

Replaces the placeholder pipe-and-dash output with Unicode
box-drawing tables for both data results and table-structure
listings, per ADR-0016.

* New `src/output_render.rs` module with `render_data_table`
  and `render_structure`. Hand-rolled to match the project's
  existing CSV/YAML pattern; ~300 lines.
* Header-only outer-frame border style: outer ┌─┐│└─┘ box +
  ├─┤ header underline, no per-row separators. NULL renders
  as `(null)`; cell newlines/tabs/control chars become
  `↵`/`→`/`·` as display-only substitutions.
* Type-aware column alignment: numeric types right-aligned,
  everything else left. `DataResult` gains a `column_types:
  Vec<Option<Type>>` field, populated from the existing
  metadata lookup at the two query sites in db.rs (no new
  query paths).
* Structure view shows Name | Type | Constraints columns;
  References / Referenced-by sections retain plain-text
  format, leaving room for the future relationship-rendering
  ADR.
* 18 new unit tests in output_render.rs (plus 4 insta
  snapshots for the canonical layouts). Existing assertions
  in app.rs and walking_skeleton.rs updated to match the new
  format.

Total: 426 passing, 0 failing, 0 skipped (up from 408).
Clippy clean.

tests/walking_skeleton.rs

@@ -298,9 +298,12 @@ fn create_table_flow_updates_tables_list_and_structure_view() {
         rendered.contains("[ok] create table Customers"),
         "output should confirm success:\n{rendered}"
     );
+    // The structure table renders one line per column; the
+    // `id` row shows both the name and its `serial` type
+    // separated by box-drawing characters.
     assert!(
-        rendered.contains("id serial"),
-        "output should show the structure with the user-facing type:\n{rendered}"
+        rendered.lines().any(|l| l.contains("id") && l.contains("serial")),
+        "output should show the id/serial column row:\n{rendered}"
     );
 }
 
@@ -339,7 +342,10 @@ fn add_column_flow_updates_structure_view() {
     });
     assert_eq!(app.current_table, Some(updated));
     let rendered = rendered_text(&mut app, &Theme::dark(), 80, 24);
-    assert!(rendered.contains("Name text"));
+    assert!(
+        rendered.lines().any(|l| l.contains("Name") && l.contains("text")),
+        "expected the Name/text column row:\n{rendered}",
+    );
 }
 
 #[test]
@@ -497,6 +503,7 @@ fn insert_flow_emits_action_and_renders_data() {
     let data = DataResult {
         table_name: "Customers".to_string(),
         columns: vec!["id".to_string(), "Name".to_string()],
+        column_types: vec![Some(Type::Serial), Some(Type::Text)],
         rows: vec![vec![Some("1".to_string()), Some("Alice".to_string())]],
     };
     app.update(AppEvent::DslInsertSucceeded {
@@ -545,6 +552,7 @@ fn show_data_for_empty_table_renders_placeholder() {
     let data = DataResult {
         table_name: "Customers".to_string(),
         columns: vec!["id".to_string(), "Name".to_string()],
+        column_types: vec![Some(Type::Serial), Some(Type::Text)],
         rows: Vec::new(),
     };
     app.update(AppEvent::DslDataSucceeded {