refactor: centralize seed hash validation

[jaideeppyne]

Closes #182.

What changed

• add one internal check_seed_hash helper that computes the expected hash once and preserves the existing invalid-data error
• use it in the Count-Min, CPC, compact Theta (v1/v2/v3), and compact Tuple deserialization paths
• remove 41 lines of repeated validation while keeping the helper crate-private

Validation

• cargo x prepare-testdata — passed (645 cross-language snapshots prepared)
• cargo x test — passed
• nightly Clippy with -D warnings — passed
• nightly rustfmt check — passed
• rustdoc with warnings denied — passed
• Taplo format check and typos — passed

cargo x lint reached and passed all repository/code checks above, but its final local-tool bootstrap could not install hawkeye 6.5.1 with the repository's Rust 1.86 toolchain because current transitive crates require Rust 1.88+; no source/license finding was reported before that environment-only bootstrap failure.

Risk and rollback

This is an internal refactor with no public API or serialized-format change. The main risk is an accidentally changed error kind/message; the helper retains ErrorKind::InvalidData through Error::deserial and the prior message text. Reverting commit 0d6570b fully rolls it back.

AI assistance

OpenAI Codex assisted with implementation and validation. I reviewed the diff and test results.

[tisonkun]

Different checks may return different error.

Perhaps a check_seed_hash(expect, actual, |expect, actual|...) help. I don't know.

[jaideeppyne]

Thanks — updated in 6b193ad. check_seed_hash now accepts an error-producing closure over (expected, actual), so each caller retains control of its error contract. In particular, the CPC path still uses its prior explicit Error::new(ErrorKind::InvalidData, ...) form, while the deserialization paths retain Error::deserial(...); the helper only owns the equality check. This also keeps the mismatch path to one seed-hash computation.

Validation after the change:

• cargo x test — passed
• cargo +nightly clippy --tests --all-features --all-targets --workspace -- -D warnings — passed
• cargo +nightly fmt --all --check — passed
• RUSTDOCFLAGS="--cfg docsrs -D warnings" cargo +nightly doc --package datasketches --all-features --no-deps — passed

[tisonkun]

Fixed up. LGTM.