feat(ui): relationships sidebar panel + schema data (#21, ADR-0046 DB2/DB4)

The left column now stacks a Tables panel over a Relationships panel.
Each relationship renders as three narrow lines — its name, then the
endpoints broken at the arrow (Customers.id -> / indented
Orders.customer_id) — ellipsized past the inner width. The panel is
content-sized within [5 rows ("(none)" when empty), half the column];
the Tables panel keeps the rest (>=3 rows). Phase C adds focus+scroll
for content beyond the cap (clipped for now).

Data path: a new worker Request::ReadAllRelationships +
Database::read_all_relationships returns full RelationshipSchema
records; the runtime posts them via a RelationshipsRefreshed event
alongside the schema-cache refresh, and the App holds them in a new
`relationships` field.

ADR deviation (recorded in ADR-0046 DB2 + index): DB2 specified this
data on SchemaCache; it lives on the App instead — SchemaCache is
walker/completion-facing and needs only relationship names (untouched),
while the full records are UI-only, so App is the cleaner home and it
avoids editing ~23 SchemaCache literals. No behavioural difference.

Tests: panel-height bounds, the three-line render, the empty "(none)"
case, a snapshot, read_all_relationships end-to-end (real DB via the
m:n junction), and the event->field handler.

docs/adr/0046-sidebar-navigation-and-responsive-input-hint.md

@@ -211,17 +211,28 @@ holds two stacked panels (DB4) instead of one.
 
 The panel needs the full `RelationshipSchema` (name, parent/child
 tables, list-based columns, on-delete/on-update actions) that the `show
-relationship` path already fetches. **`SchemaCache` is *extended*, not
-retyped:** its existing `relationships: Vec<String>` is left as-is
-(`completion.rs` borrows it as `&Vec<String>` via
-`IdentSource::Relationships` for relationship-name completion, and
-several test fixtures construct it) and a **new field
-`relationship_details: Vec<RelationshipSchema>`** is added alongside,
-populated by the same cache refresh that runs on schema change (the
-refresh is taught to query relationship detail, which today it does not
-— it only lists names). Retyping the existing field would break the
-completion borrow and the fixtures; adding a field is the
-zero-ripple change.
+relationship` path already fetches.
+
+**Data home — `App`, not `SchemaCache` (revised at implementation,
+2026-06-10).** The design first proposed an additive
+`SchemaCache.relationship_details: Vec<RelationshipSchema>` field.
+Implementation revised this to a **parallel `App.relationships:
+Vec<RelationshipSchema>`** field for two reasons: (1) `SchemaCache` is
+*walker/completion-facing* — it needs only relationship **names**
+(unchanged in `SchemaCache.relationships`, still borrowed as
+`&Vec<String>` by `IdentSource::Relationships`); the full records are
+**UI-only**, so `App` is the architecturally correct home, mirroring
+`app.tables` (which the items panel already reads alongside the cache).
+(2) Adding a field to `SchemaCache` would force edits to ~23 full
+struct literals across the test suite, whereas `App` gains one field.
+The /runda guard it answered — *don't break completion by retyping
+`relationships`* — is fully honoured either way. Delivery: a worker
+`Request::ReadAllRelationships` (→ `Database::read_all_relationships`,
+returning `Vec<RelationshipSchema>` via the existing
+`read_all_relationships(conn)`); the runtime's `refresh_schema_cache`
+posts a new `AppEvent::RelationshipsRefreshed` alongside
+`SchemaCacheRefreshed`, and the `App` stores it. No behavioural
+difference from the original design.
 
 The panel has **two display states** keyed off focus (DC2):
 
@@ -357,10 +368,11 @@ silently edit an invisible buffer.
 - Per-panel scroll offsets for the Tables and Relationships panels, each
   clamped against a renderer-reported viewport (DC3), mirroring
   `output_scroll` / `note_output_viewport`.
-- `SchemaCache` gains **`relationship_details: Vec<RelationshipSchema>`**
-  (DB2) — *additive*; the existing `relationships: Vec<String>` (names,
-  used by `completion.rs` `IdentSource::Relationships`) is unchanged. The
-  cache refresh is extended to populate the new field.
+- **`App.relationships: Vec<RelationshipSchema>`** (DB2) — the full
+  relationship records for the sidebar panel, delivered by
+  `AppEvent::RelationshipsRefreshed` from the runtime's schema refresh.
+  `SchemaCache.relationships: Vec<String>` (names, for completion) is
+  unchanged. (See DB2 for why this lives on `App`, not `SchemaCache`.)
 
 **Render-time derived** (pure functions of `frame.area()` + cached
 counts — *not* stored fields; this keeps the pure-render invariant and
@@ -458,11 +470,11 @@ Phase A:
   scrolls.
 
 Phase B:
-- **Schema-cache enrichment:** after each schema mutation the cache
-  carries full `relationship_details` (name, parent/child, columns,
-  actions) *and* the existing `relationships` names; `completion.rs`
-  `IdentSource::Relationships` still resolves names (the additive field
-  did not disturb it).
+- **Relationship data path:** `Database::read_all_relationships`
+  returns full records through the worker thread (integration test, real
+  DB via an m:n junction); `AppEvent::RelationshipsRefreshed` populates
+  `App.relationships`; `SchemaCache.relationships` names are undisturbed
+  (completion still resolves them).
 - **Relationships panel render (Tier-2):** empty → a single `None` line
   at the 5-row floor; the unfocused narrow format (name + arrow-break,
   ellipsis past inner width); a compound endpoint pair arrow-breaks