Add per-kilometre breakdown chart for most recent run

[c-wilkinson]

Add a detailed summary visual for the most recent run.

The chart should present key per-kilometre metrics as aligned multi-line plots to allow easy comparison across the run:

• Pace
• Cadence
• Stride length
• Elevation

The visual should be produced as a single clean PNG, suitable for README embedding, and implemented as a dedicated graph module that reuses the existing visualisation utilities where possible.