feat: compound-FK bus routing + complete V1 relationship visualization (ADR-0044)

Completes requirement V1. A compound (multi-column) FK now routes a
bus connector — each paired endpoint's stub merges into a shared
vertical channel that splits to the other side — plus an explicit
"(a, b) ▶ P.(x, y)" pairing line; the bus generalises the single-column
jog (reproducing it exactly, so prior snapshots are unchanged).
Self-referential FKs render as two same-named boxes.

- output_render.rs: gutter_seg routes all endpoint pairs via a
  junction() bus; pairing line for compound FKs; compound, self-ref,
  and compound-from-data (build_diagram_table glue) tests + snapshots
- compound_fk.rs: worker test that show_relationship carries both
  paired column lists into the diagram payload
- db.rs: document do_show_one's now-app-superseded relationship prose
  branch (retained as a worker-API/text fallback; could back a future
  non-visual display option, cf. ADR-0044 OOS-7)

Second /runda pass over the implementation: confirmed ADR-compliance,
UTF-8/byte-range safety, and edge-case routing. The ADR §3 last-resort
helper line was considered and rejected (vertical fallback + ratatui
truncation cover all realistic cases). ADR-0044 marked implemented;
requirements.md V1 -> [x].

Full suite 2207 pass / 0 fail / 1 ignored; clippy nursery clean.

docs/requirements.md

@@ -423,20 +423,24 @@ since ADR-0027.)
 
 ## Visualizations
 
-- [/] **V1** Single-element views render in the output pane: a
+- [x] **V1** Single-element views render in the output pane: a
   selected table as its structure (columns, types, keys,
   constraints); a selected relationship as two tables joined by
   a line.
-  *(Partial, verified 2026-06-07: the **table-structure** half is
-  done — `output_render.rs:82-180` renders columns / types /
-  constraints / indexes in a box-drawing table, with relationship
-  metadata as `References:` / `Referenced by:` prose
-  (`A.col → B.col`). The **relationship-as-line-art** half — two
-  tables drawn side by side with a connecting line — is **not
-  implemented** (deferred per `output_render.rs` §5 OOS-1, ADR
-  pending). This is the relationship-visualisation piece that has
-  been repeatedly pushed away; it is the substantive open part of
-  V1. Selection-nav and the broader journal direction live in V4.)*
+  *(Done 2026-06-10 — **ADR-0044**. The table-structure half shipped
+  earlier; the **relationship-as-line-art** half (ADR-0016 OOS-1) now
+  renders as **Style-A two-table connector diagrams** wherever a
+  relationship is the subject: `show relationship <name>` (full
+  structure boxes), `show table <T>` and `add`/`drop relationship`
+  echoes (focal box + compact stacked diagrams). Child-left /
+  parent-right, `n…1` cardinality, referential actions, bold title
+  rows; rendered App-side and **width-aware** (side-by-side ↔ vertical
+  fallback). **Compound** FKs route a shared bus + an explicit
+  `(a, b) ▶ P.(x, y)` pairing line; **self-referential** FKs draw two
+  same-named boxes. Incidental DDL echoes keep the prose form (the
+  "relationship-relevant" reach). The §3 last-resort helper line was
+  considered and rejected. Two `/runda` passes (design + implementation).
+  Selection-nav and the broader journal direction remain in V4.)*
 - [/] **V2** SQL query results render as a dynamic table view in
   the output pane, with multiple result tabs supported.
   *(Partial, verified 2026-06-07: the **table view** is done —