fix: classify CPC image errors and clarify intersection naming

[tisonkun]

Summary

• classify out-of-range lg_k and first_interesting_column values read from CPC images as InvalidData in both CpcSketch and CpcWrapper
• cover both valid lg_k boundaries and the first-interesting-column boundary with regression tests
• keep all public sketch-materialization APIs named to_sketch, while renaming the internal IntersectionState::result helper to to_compact_parts because it returns intermediate CompactSketchParts rather than a sketch
• record the observable CPC error-kind correction in the changelog

Rationale

InvalidArgument describes a bad direct API argument. These values come from a serialized image, so an out-of-range value means the image is malformed and should use InvalidData consistently with the other deserialization checks.

The public result-extraction audit found no remaining result method: CPC, HLL, Theta, and Tuple operators already use to_sketch. The only result helper was crate-internal and did not produce a sketch, so to_compact_parts states its behavior more accurately.

Validation

• cargo x check
• cargo x test
• cargo x lint