Survive a busy engine: retry, idempotent submit, resilient waiting

A driver script died of one slow answer: httpx.ReadTimeout out of
RunHandle.refresh() with a 30 s read timeout and no retry anywhere, which
cost a sweep 78 of its 84 runs.

- Split the timeout (5 s connect, 120 s read): a wrong URL fails at once,
  and a busy engine gets longer than the slowest thing it does on purpose
  (a 60 s compile, a 15 s rebuild wait).
- Retry idempotent calls three times on a transport error or 502/503/504.
  503 is the engine's own "ask again" — it is what RebuildBusy answers.
- Submit carries a key the engine stores with the run, so a retry after a
  timeout returns that run instead of starting a second. A sweep keys every
  entry, so a half-created one recreates only what is missing.
- wait() and --follow tolerate five failed polls in a row; a 404 still stops
  at once, because that is an answer rather than a gap.
- CLI says "engine not answering" and names the run still on the engine,
  instead of printing a traceback.
- runs: clamp the params column to 80 characters; events() takes the
  flow/since/until the endpoint already had; RunHandle.failures answers
  "what killed this run" from the run's own node rows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/getting-started/data-science.md

@@ -449,7 +449,33 @@ curl -s "$FLUKSIO/runs/<id>/metrics?name=train.loss" -H "Authorization: Bearer $
 The run carries its parameters, a digest of them, the seed, its result, how
 long each node took, what it logged, every artifact it produced, and both
 commits — the flow store's and your repository's. That is the answer to "what
-was the learning rate on the run that got 94%?".
+was the learning rate on the run that got 94%?". When a run failed,
+`run.failures` is the node that did it, with its traceback and its logs.
+
+### When the engine is busy
+
+A driver script outlives the engine being slow, because a sweep is hours long
+and a blip is seconds:
+
+- **Reads are repeated.** Every GET, and the writes that mean the same thing
+  twice, are tried up to three more times on a transport error or a `502`,
+  `503` or `504`, waiting 1 s, 2 s, then 4 s. `Client(retries=0)` turns that
+  off.
+- **Submitting is safe to repeat.** Each `submit()` mints a key the engine
+  stores with the run, so an attempt that timed out on the way back is
+  answered with the run it already made rather than starting a second one. A
+  sweep keys every entry, so retrying a half-created one creates only what is
+  missing.
+- **Waiting tolerates a gap.** `wait()` and `--follow` survive five failed
+  polls in a row before giving up. An answer is not a gap: a `404` for a run
+  that is gone stops immediately.
+- **Timeouts are split** — 5 s to connect, 120 s to read, so a wrong URL fails
+  at once while a busy engine is given longer than the slowest thing it does
+  on purpose. `Client(timeout=…)` takes a number or an `httpx.Timeout`.
+
+A command that still cannot reach the engine says
+`fluksio: engine not answering (…)` and names the run it started, which is
+running on the engine regardless — `fluksio runs` finds it again.
 
 ## Sweep it