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/snapshots/rdbms_playground__output_render__tests__render_data_table_basic_shape.snap

@@ -0,0 +1,10 @@
+---
+source: src/output_render.rs
+expression: "render_data_table(&data).join(\"\\n\")"
+---
+┌────┬───────┬──────────────┐
+│ id │ Name  │ Email        │
+├────┼───────┼──────────────┤
+│  1 │ Alice │ a@example.io │
+│  2 │ Bob   │ b@example.io │
+└────┴───────┴──────────────┘

src/snapshots/rdbms_playground__output_render__tests__render_data_table_empty_rows_shows_no_rows_marker.snap

@@ -0,0 +1,9 @@
+---
+source: src/output_render.rs
+expression: out
+---
+┌───────────┬──────┐
+│        id │ Name │
+├───────────┼──────┤
+│ (no rows) │      │
+└───────────┴──────┘

src/snapshots/rdbms_playground__output_render__tests__render_structure_basic.snap

@@ -0,0 +1,12 @@
+---
+source: src/output_render.rs
+expression: "render_structure(&desc).join(\"\\n\")"
+---
+Customers
+┌───────┬────────┬─────────────┐
+│ Name  │ Type   │ Constraints │
+├───────┼────────┼─────────────┤
+│ id    │ serial │ PK          │
+│ Name  │ text   │ NOT NULL    │
+│ Email │ text   │             │
+└───────┴────────┴─────────────┘

src/snapshots/rdbms_playground__output_render__tests__render_structure_with_relationships.snap

@@ -0,0 +1,12 @@
+---
+source: src/output_render.rs
+expression: out
+---
+Customers
+┌──────┬────────┬─────────────┐
+│ Name │ Type   │ Constraints │
+├──────┼────────┼─────────────┤
+│ id   │ serial │ PK          │
+└──────┴────────┴─────────────┘
+Referenced by:
+  Orders.cust_id → id  (cust_orders, on delete cascade, on update no action)