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.

src/db.rs

@@ -134,12 +134,20 @@ pub enum DbError {
     Io(String),
 }
 
-/// Result of a query / show-data call (schema-less display rows).
-/// `None` cells render as NULL; `Some(s)` renders as the string.
+/// Result of a query / show-data call.
+///
+/// `None` cells render as NULL; `Some(s)` renders as the
+/// string. `column_types` carries the user-facing type per
+/// column (per ADR-0016 §2): the renderer uses it for
+/// alignment, and future work uses it for type-aware cell
+/// styling. `None` only for the edge case of a
+/// foreign-attached database we did not create — not
+/// achievable in normal use.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct DataResult {
     pub table_name: String,
     pub columns: Vec<String>,
+    pub column_types: Vec<Option<Type>>,
     pub rows: Vec<Vec<Option<String>>>,
 }
 
@@ -1970,6 +1978,7 @@ fn query_rows_by_rowid(
         return Ok(DataResult {
             table_name: table.to_string(),
             columns: column_names,
+            column_types,
             rows: Vec::new(),
         });
     }
@@ -2016,6 +2025,7 @@ fn query_rows_by_rowid(
     Ok(DataResult {
         table_name: table.to_string(),
         columns: column_names,
+        column_types,
         rows,
     })
 }
@@ -2355,6 +2365,7 @@ fn do_query_data(conn: &Connection, table: &str) -> Result<DataResult, DbError>
     Ok(DataResult {
         table_name: table.to_string(),
         columns: column_names,
+        column_types,
         rows,
     })
 }