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/README.md

@@ -43,7 +43,7 @@ src/
   lib/trail.svelte.ts     the walked path; mirrored into the `t` query param
   lib/kinds.ts            kind glyph letters
   lib/map-model.ts        the Map's deterministic layered layout (pure)
-  lib/flow-model.ts       the Flow strip's card/link geometry — a DAG (pure)
+  lib/flow-model.ts       the Flow strip's card/link geometry + the end cap — a DAG (pure)
   lib/filecode-model.ts   the whole-file view: fixed line height, arcs, paging (pure)
   lib/live.svelte.ts      /api/events: two counters every screen refreshes from
   lib/toast.svelte.ts     the one transient note ("Index updated · reloaded")
@@ -68,6 +68,28 @@ announce the project to a font CDN.
 | `#/flow?symbols=a,b,c` | flow strip — `codegraph_explore`'s own question |
 | `#/flow?t=<trail>` | flow strip — the trail you walked, read as a flow |
 
+## Where the graph stops
+
+A flow that does not reach everything it was asked about carries a
+`boundary` on the wire, and `buildFlowLayout` turns it into an extra 240px node
+one column past the symbol the path stopped at, joined by a dotted `2 4` link
+labelled "end of static path" that deliberately has **no arrowhead** — an arrow
+would point at a continuation, and the absence of one is the finding.
+
+Two rules hold it together:
+
+- **The cap's height is arithmetic, like a card's.** `endCapText()` builds every
+  sentence the cap shows and `endCapHeight()` measures them; the component then
+  renders exactly what was measured. Change the wording in one and the other
+  moves with it — they are the same function read twice.
+- **One cap per stopping symbol, not per flow.** Two paths that run out at the
+  same place ran out for the same reason, and two caps side by side would read
+  as two different findings.
+
+The verdict itself is not computed here or in the server: it is
+`findDynamicBoundaries` in `src/graph/dynamic-boundary-report.ts`, the same
+detector `codegraph_explore` announces boundaries with.
+
 ## Live updates
 
 The viewer never polls. `lib/live.svelte.ts` holds one `EventSource` on