refactor: align internal symbol visibility

[tisonkun]

Summary

• let private and crate-private module boundaries control the reachability of shared implementation items instead of repeating pub(super) or pub(crate) on each symbol
• make same-module helpers private and remove test-only field accessors
• retain restricted visibility where re-exported public types or the public thetacommon module require a real API boundary
• document the visibility convention in CONTRIBUTING.md

The audit reduces pub(super) occurrences from 143 to 31 and pub(crate) occurrences from 20 to 13. The remaining restricted items all protect externally reachable types or modules.

Public API

No public API or behavior changes are intended. The all-feature rustdoc public path set is unchanged, and patch-level cargo-semver-checks reports no required version update.

No changelog entry is needed because this is an internal refactor with no user-visible behavior change.

Testing

• cargo x check
• cargo x test
• cargo x lint
• cargo +stable semver-checks check-release --baseline-rev origin/main --package datasketches --all-features --release-type patch (223 checks passed)