feat(llm): make runs resilient and reproducible on slow backends

Retry with exponential backoff and jitter, and honour Retry-After. Retry empty-choice responses and HTTP 408.
Add max_retries, retry_timeouts, model_timeouts and model_concurrency settings for slow local models.
Add opt-in SSE streaming with an idle timeout. A hung server fails early, and a slow but active generation continues.
Send an optional seed and record the backend fingerprint of each answer.
Stamp saved artifacts with run metadata and write them atomically. Warn in compare when two runs differ.
Save partial results on Ctrl-C. Turn an evaluator panic into an error row.
Reject duplicate models and non-http endpoints in the config.

README.md

@@ -75,6 +75,8 @@ Both `list` and `run` accept:
   `60s`).
 - `--out FILE` - write the run as a JSON artifact (raw per-attempt results
   plus the per-test discrimination rollup) for later `llmtest compare`.
+- `--seed N` - send `seed: N` with every request (overrides the config's
+  `seed`). Whether the backend honors it is backend-specific.
 - `--quiet` - suppress progress output on stderr. Progress is on by
   default: a run can issue hundreds of requests at 30-50s each, and a
   silent CLI reads as a hang.