GET /runs/{id}/nodes/{node}/outputs serves what a node returned

The stage cache's stored outputs, as kept (up to 256 kB of JSON). A node
that kept none answers 404 saying why. Listed in the API docs.

docs/code/api.md

@@ -128,6 +128,7 @@ values. You can publish only to declared messages, just as dashboard controls do
 | `GET` | `/runs` | query history |
 | `GET` | `/runs/overview` | summarize runs per flow |
 | `GET` | `/runs/{id}` | read parameters, results, nodes, and artifacts |
+| `GET` | `/runs/{id}/nodes/{node}/outputs` | read what one node returned |
 | `POST` | `/runs/{id}/cancel`, `/runs/{id}/retry` | cancel, or create a retry |
 | `DELETE` | `/runs/{id}` | delete a finished run and its records |
 | `DELETE` | `/runs?flow={name}` | delete a flow's runs; refused while any are active |
@@ -144,6 +145,10 @@ History filters include `flow`, `status`, `group`, `digest`, `since`,
 `before`, `limit`, and `offset`. Rows arrive newest first; use the last
 row's `created_at` as `before` for stable pagination.
 
+A node's outputs come from the stage cache, so only a node that succeeded
+and may be cached has them, up to 256 kB of JSON. Any other node answers
+`404`.
+
 Exports also accept `ids` and `until`, with `format=csv` or `jsonl`.
 The metrics export is a long table; the runs export is one row per run.
 Use `params` and `metrics` to choose columns, including dotted paths into