Improve storm cell detection quality

[danielway]

Scope

Improve reflectivity segmentation within each radar sweep so its output is a reliable observation layer for volume aggregation. Detection supports reflectivity only.

Work

• Establish a checked-in, curated corpus of representative sweeps with reviewed expected observations.
• Reduce false merges, false splits, and noise detections with documented thresholds and morphology/quality rules.
• Preserve azimuth-wrap correctness and define behavior for incomplete live sweeps.
• Emit geometry, intensity summaries, source sweep identity, and quality metadata needed by Aggregate reflectivity observations into volume storm cells #178.

Non-goals

• Combining elevations, tracking through time, estimating motion, or changing measurement interaction.
• Supporting non-reflectivity products.

Acceptance criteria

• Detection is unavailable for non-reflectivity products with clear core/view state.
• Curated fixtures cover isolated cells, nearby cells, noise, false merge/split cases, azimuth wrap, and incomplete live data.
• Thresholds, assumptions, eligible sweep states, and known limitations are documented.
• Output is deterministic and contains the source/geometry metadata required for vertical aggregation.
• Pure decision and algorithm tests assert reviewed outcomes without egui or browser dependencies.

Parent: #33; blocks #178.