Add offset top x-axis to runtime layout plot

[Copilot]

The runtime layout figure needed to match the provided reference while adding a second x-axis. The new top axis now uses the same core-count units as the bottom axis, offset to start at 4096.

• Scope

  • Updated pace/runtime_layout.py to support a secondary top x-axis aligned with the existing processor-core axis.
  • Kept the bottom axis as the canonical processor range (0..4096) and corrected primary tick spacing.

• Top-axis behavior

  • Replaced the GPU-conversion mapping with an offset mapping:
    • top = bottom + 4096
    • bottom = top - 4096
  • Configured top ticks to display 4096, 5120, 6144, 7168, 8192.

• Labeling

  • Clarified axis labels for readability:
    • Bottom: Processor Cores
    • Top: Processor Cores (+4096 offset)

ax_top = ax.secondary_xaxis(
    "top",
    functions=(
        lambda cores: cores + TOTAL_PROCS,
        lambda top_units: top_units - TOTAL_PROCS,
    ),
)
top_ticks = [4096, 5120, 6144, 7168, 8192]
ax_top.set_xticks(top_ticks)