feat: add Display summaries for HLL and CPC

[zhangxinyao88]

Part of #212.

Summary

• implement Display for HllSketch, HllUnion, CpcSketch, and CpcUnion
• keep the output summary-only, without raw register or coupon details
• add coverage for empty and populated sketches and update the changelog

Example output:

HLL Sketch Summary:
  lg config k       : 12
  target type       : Hll8
  current mode      : List
  lower bound       : 0
  estimate          : 0
  upper bound       : 0

CPC Sketch Summary:
  flavor            : Empty
  lg k              : 11
  merged            : false
  lower bound       : 0
  estimate          : 0
  upper bound       : 0
  num coupons       : 0

Testing

• cargo x check
• cargo x test
• cargo +nightly clippy --tests --all-features --all-targets --workspace -- -D warnings
• cargo +nightly fmt --all --check
• cargo +nightly doc --package datasketches --all-features --no-deps

[tisonkun]

I think you can either just use the Debug impl for flavor here or define a pub const fn as_str for Flavor`.

[zhangxinyao88]

Updated this to use the existing Debug implementation for Flavor.

[tisonkun]

Why NumStdDev::One? Could you point out the referenced implementation?

[zhangxinyao88]

Good point. The Java HLL summary uses one standard deviation, but the Java CPC summary does not include bounds. I removed the CPC lower and upper bounds instead of introducing that choice here.

[tisonkun]

Ditto

[zhangxinyao88]

Removed this together with the lower bound, since the Java CPC summary does not include either bound.

[tisonkun]

You may use insta for snapshot testing.

However, I have more to consider here now. Let me comment on the issue.

[zhangxinyao88]

Updated the exact Display assertions to use insta inline snapshots. I kept the populated cases as focused assertions so we do not snapshot estimator values unnecessarily.