feat(ui): the type hierarchy — what a type is built on, and what dispatches through it (CG-58)

A vertical tree above the members outline for classes, interfaces, structs,
traits, protocols, enums, unions and type aliases: ancestors above (the whole
chain, not just the direct parent), the focus in accent, subtypes below indented
per level. `extends` draws solid, `implements` dashed; a synthesized edge — Go's
implicit interface satisfaction — draws dashed wider and carries the site it was
wired at, so a relation the resolver inferred never reads like one the source
wrote down. For an interface the fan below IS the set of runtime targets a call
can land on, and a type with eight or more implementers leads with that in a
sentence. Members that redeclare an ancestor's are marked in the outline.

The walk lives in `src/graph/type-hierarchy.ts`, following CG-50/CG-51: shared
computation in `src/graph/`, presentation in the caller. Its `countImplementers`
is now also what `ToolHandler.buildPolymorphicBoundaries` counts with, so "N
types implement X" is the same N whether an agent reads it or a person does.
`/api/node` carries the block as `hierarchy` rather than a second endpoint —
it is part of the Symbol view's first paint, and gated to types, so a function
costs one kind test.

Layout is arithmetic (24px rows, 22px indent, orthogonal connectors computed
from the two): no ResizeObserver, same payload → same picture. The header's
`extends X` / `implemented by …` chips are suppressed while the tree is on
screen — two renderings of one relation in one column is how a reader ends up
trusting neither.

`TypeHierarchy` is exported from `@colbymchenry/codegraph-ui` and takes its data
as a prop, so a host holding a `WireSymbolPayload` renders it without a second
read.

ui/src/index.ts

@@ -111,6 +111,8 @@ export { default as DriftBanner } from './components/DriftBanner.svelte';
 export { default as KindGlyph } from './components/KindGlyph.svelte';
 /** Copy image / download SVG for a Flow strip or a Map layout. */
 export { default as ExportButtons } from './components/ExportButtons.svelte';
+/** Ancestors up, subtypes down, and the fan an interface call dispatches into. */
+export { default as TypeHierarchy } from './components/symbol/TypeHierarchy.svelte';
 
 /* ------------------------------------------------------------- the state -- */
 
@@ -174,6 +176,21 @@ export type {
   FlowLinkLayout,
 } from './lib/flow-model';
 
+export {
+  buildHierarchyModel,
+  connectorPath,
+  visibleHierarchy,
+  HIER_FOLD_AT,
+  HIER_INDENT,
+  HIER_PORT_X,
+  HIER_ROW_H,
+} from './lib/hierarchy-model';
+export type {
+  HierarchyConnector,
+  HierarchyModel,
+  HierarchyRow,
+} from './lib/hierarchy-model';
+
 export { buildMapLayout, isEdgeVisible, moduleMetaLabel } from './lib/map-model';
 export type {
   MapEdgeLayout,