feat(ui): where the graph stops — the Flow strip's dynamic-dispatch end cap (CG-51)

A flow that does not reach what it was asked about now ends in a cap instead
of in silence: the dispatch form that ended it, the line, the static key when
the source spells one out, the candidate runtime targets as clickable rows,
and the name-only matches under 0.6 the search refused to follow. A flow that
does reach its destination never shows one.

The verdict is lifted out of `ToolHandler` into
`src/graph/dynamic-boundary-report.ts` and both callers render it —
`codegraph_explore`'s prose and `/api/flow`'s `WireFlowBoundary` — the same
move `named-symbol-flow.ts` made for the path finder, and for the same reason:
a reader holding the strip and the MCP answer must not be told two different
things. The explore prose is unchanged, byte for byte.

When nothing connects at all and a dispatch site explains why, the strip is
that site: one card opened at the line where the static path ends, plus the
cap. When nothing explains it, no stopping point is invented.

ui/src/lib/api.ts

@@ -459,11 +459,50 @@ export interface WireFlowHop {
   source: WireFlowSource | null;
 }
 
+/** One plausible runtime target of a keyed dispatch — a clickable cap row. */
+export interface WireBoundaryCandidate {
+  node: WireNodeRef;
+  display: string;
+  named: boolean;
+}
+
+/** A dynamic-dispatch site: the form, the key when it is visible, the targets. */
+export interface WireBoundarySite {
+  form: string;
+  label: string;
+  snippet: string;
+  line: number;
+  key: string | null;
+  keyIsType: boolean;
+  moreSites: number;
+  candidates: WireBoundaryCandidate[];
+  candidateNote: string | null;
+}
+
+export interface WireFlowContinuation {
+  node: WireNodeRef;
+  line: number | null;
+  confidence: number | null;
+}
+
+/** Where the graph stops — the strip's end cap (design spec §3.5). */
+export interface WireFlowBoundary {
+  node: WireNodeRef;
+  sites: WireBoundarySite[];
+  uncertain: WireList<WireFlowContinuation>;
+  further: WireList<WireFlowContinuation>;
+  missed: WireNodeRef[];
+}
+
 export interface WireFlow {
   id: string;
   /** "execute → rowToFileRecord", for the header's flow picker. */
   label: string;
   hops: WireFlowHop[];
+  /** Null on a flow that reaches everything it was asked about. */
+  boundary: WireFlowBoundary | null;
+  /** One card at the dispatch site, not a path: the answer ran out here. */
+  partial: boolean;
 }
 
 export interface WireFlowAmbiguity {