feat: charts with uneven x and step series #86

What

Charts assume equally spaced f32 samples. Simulation and profiling data is not.

Acceptance

• Series with explicit x positions, including uneven spacing
• A step series (value holds until the next sample)
• u64 and f64 values without precision loss in axes and readouts
• Bars with a no-gap option
• Hover readout and a vertical cursor line that can be driven by the caller
• Log scale on either axis