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.

CHANGELOG.md

@@ -66,6 +66,12 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
   Nothing changes for `codegraph ui` itself — it is the same viewer, now the library's first user. The package is prepared, not yet on npm.
 
+- **See what a type is built on, and what is built on it, in `codegraph ui`.** Open a class, interface, struct, trait or enum and a small tree now sits above its members: what it extends and implements, going all the way up rather than stopping at the direct parent, and everything that extends or implements it, going down. Inheritance is drawn with a solid line and implementation with a dashed one, so the two never read as the same relationship, and every row opens the type it names.
+
+  For an interface, the list below it is the answer to a question source code cannot give you: a call through that interface can land on any of them, and where there are enough of them to make a static answer meaningless the block says so in a sentence. Go's implicit interface satisfaction is included — a struct that satisfies an interface without either file mentioning the other appears in the fan, marked as matched by CodeGraph rather than written down, along with the line it was matched at. Long fans fold behind a "+N more implementations" button rather than being cut off, and if there is more below than was walked the tree says that too.
+
+  Members that redeclare something from a type above are marked in the outline ("overrides Base", or "satisfies Clock" for an interface), so a 40-member class shows at a glance which parts are its own and which are a contract it is filling. The number of implementations shown here is the same number `codegraph_explore` reports to your agent when it announces an interface dispatch.
+
 ### Fixes
 
 - Fixed a long-running `codegraph ui` session serving a symbol that a sync had already deleted. The viewer keeps one connection to your index open, and its in-memory lookup didn't notice when another process — your agent's sync, or `codegraph sync` — rewrote the file underneath it, so a symbol screen could keep showing a body with no callers while search correctly reported it had moved. Because a symbol's identity includes the line it starts on, this happened after almost any edit above it.