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.

__tests__/ui-server-api.test.ts

@@ -603,6 +603,18 @@ describe('GET /api/node/<id>', () => {
   });
 });
 
+describe('GET /api/node/<id> — the type hierarchy block', () => {
+  it('is null for a function, so the block costs a plain symbol nothing', async () => {
+    const body = await getJson(`/api/node/${await idOf('hot', 'function')}`);
+    expect(body.hierarchy).toBeNull();
+  });
+
+  it('is null for a class with nothing above or below it', async () => {
+    const body = await getJson(`/api/node/${await idOf('Cache', 'class')}`);
+    expect(body.hierarchy).toBeNull();
+  });
+});
+
 describe('GET /api/node/<id> — the busiest symbol', () => {
   it('caps the caller list, keeps the true total, and stays fast', async () => {
     const hotId = await idOf('hot', 'function');