The override sidecar now accepts { patches: [{ at, entry }] } alongside the
legacy whole-script ReplayEntry[] replacement: the JSONL-derived script is
kept and only the named call indexes are swapped (at == length appends, for
a retry attempt following an injected transient throw). Out-of-range or
non-integer indexes fail loud with the derived length in the diagnostic.
This is the mock-LLM error capability the web e2e scenarios drive: 'call N
throws AUTH/SERVER, everything else replays as recorded'.
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