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.

docs/adr/README.md

@@ -21,3 +21,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0013 — Relationships, naming, and the rebuild-table strategy](0013-relationships-and-rebuild-table.md)
 - [ADR-0014 — Data operations, value literals, and the auto-show pattern](0014-data-operations-and-value-model.md)
 - [ADR-0015 — Project storage runtime](0015-project-storage-runtime.md)
+- [ADR-0016 — Pretty table rendering for data and structure views](0016-pretty-table-rendering.md)