feat(ci): use 6-way slicing based on benchmark results
Benchmarked 4/5/6/7/8 slices with LPT duration-balanced distribution:
- 4 slices: 4.8m wall, 135s spread
- 5 slices: 3.4m wall, 46s spread
- 6 slices: 3.3m wall, 26s spread ← optimal
- 7 slices: 3.9m wall, 109s spread
- 8 slices: 3.7m wall, 96s spread
6 slices is the sweet spot: lowest wall time, tightest spread.
7+ gets slower due to per-slice startup overhead dominating.
Also removes benchmark branch markers from save-durations condition.
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