feat: connect null gaps in KPI sparkline

[djbarnwal]

• Extract bridgeSmallGaps and computeSegments into a shared sparse-data-utils.ts module, making the logic generic over any data type via valueAccessor/cloneWithValue callbacks
• Reuse the shared utility in the KPI sparkline (Chart.svelte) to bridge small null gaps with linear interpolation, matching the existing behavior in explore dashboard time-series charts
• Add bridgedSegments to BridgeResult so callers no longer need a separate computeSegments call on the output
• Use SVG clip paths in the sparkline to prevent area fill from rendering across gaps
• Show isolated singleton points as dots when they have no adjacent non-null neighbors
  https://linear.app/rilldata/issue/APP-571/spark-charts-on-canvas-are-just-displaying-singular-data-points

Before

After

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!