Aggregate reflectivity observations into volume storm cells

[danielway]

Build multi-elevation storm cells from the reflectivity observations produced by #164.

Scope

• Associate spatially connected observations across available reflectivity elevations in one radar volume.
• Represent each cell by its ground footprint and reflectivity-weighted ground-footprint centroid.
• During live collection, update cells as elevations arrive and mark them provisional.
• Finalize the volume-cell set when the volume completes.
• Preserve enough geometry and provenance for temporal tracking and inspection.

Non-goals

• Associating cells across volumes or estimating motion.
• Distance-tool interaction.
• Supporting non-reflectivity products.

Acceptance criteria

• Representative vertically coherent observations aggregate into one volume cell according to documented rules.
• Spatially distinct observations remain separate, including cases with overlap at only some elevations.
• Provisional cells update deterministically as live elevations arrive and are visibly distinguishable from finalized cells.
• Finalization is deterministic and does not retain stale observations from an earlier volume.
• Every cell exposes its contributing sweeps, ground footprint, centroid, vertical extent, intensity summary, and provisional/final state.
• Curated fixtures cover isolated cells, nearby cells, vertical gaps, incremental updates, and completion.
• Aggregation decisions are pure and headlessly tested.

Depends on #164. Parent: #33