Stabilize live power chart updates

[ridecontrol]

What changed

• replace global even chart sampling with a bounded fixed-bucket first/min/max/last envelope
• retain extrema for every plotted metric, including short power spikes
• keep completed buckets stable while the current live bucket receives new readings
• use elapsed time as the durable chart datum identity
• explicitly disable chart transition animation for live updates

Why

The prior sampler recalculated every plotted source index as a session grew. On longer rides, noisy power readings repeatedly entered and left the rendered set, making the power line and its scale jump even though the underlying ride history had not changed.

User impact

Live power remains responsive to new readings without rewriting the already-recorded portion of the chart. Power peaks remain visible and the chart keeps its bounded rendering cost.

Validation

• bun run ci
• 375 tests passed
• TypeScript, Biome, Tailwind diagnostics, and production build passed