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- db.rs: document do_show_one's now-app-superseded relationship prose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.

src/db.rs

@@ -6010,6 +6010,15 @@ fn do_show_list(
 /// labelled block, or a friendly "no such item" line. `Tables` is
 /// never routed here (the table singular is `ShowTable`); the
 /// defensive arm keeps the match total without a panic.
+///
+/// **The `Relationships` arm is superseded for the app by
+/// `do_show_relationship` (ADR-0044): the runtime reroutes a named
+/// `show relationship` to the structured diagram path, so this prose
+/// form is no longer shown to users.** It is retained — reachable via
+/// the `Database::show_list` worker API and covered by a worker test —
+/// as a text fallback that could back a future non-visual display
+/// option (cf. ADR-0044 OOS-7's relationship-display setting). The
+/// `Indexes` arm remains live (`show index <name>` still routes here).
 fn do_show_one(
     conn: &Connection,
     kind: crate::dsl::command::ShowListKind,