fix(main): preserve per-run dot output under --simulation_first_traces

[jp-fizzbee]

Each captured run wrote to a shared outDir/graph.dot path, so all but the last overwrite silently. Only the final write survived and the flag's intent — capture the first N runs as separate samples — was unreachable. Reproduced on the salon spec with --max_runs 1000 --simulation_first_traces 5: five "Writen graph dotfile" log lines all pointed at the same path and the on-disk content was the fifth run only.

Thread a fileName argument through writeDotFileIfNeeded:

• non-simulation call site keeps "graph.dot" (unchanged behavior)
• simulation in-loop site uses "graph_run_.dot" so the first N runs land in distinct files
• post-loop "last run" write keeps the canonical "graph.dot" name so any downstream tooling reading the well-known path keeps working

Backward compatible: anything that consumed graph.dot keeps finding it (the post-loop write now fires unconditionally in simulation mode, including the edge case where --max_runs <= --simulation_first_traces and the canonical name would otherwise not exist). New consumers can also read graph_run_*.dot for the captured first-N samples.

Verified on the salon spec for both --max_runs 5 --simulation_first_traces 5 (regression case, all 5 captures + graph.dot) and --max_runs 1000 --simulation_first_traces 5 (5 captures + graph.dot from a non-captured later run).