docs: add missing_docs lint and close all rustdoc coverage gaps

[harsh04044]

Follow-up to PR #67.

Added #![warn(missing_docs)] to lib.rs as a permanent regression guard - any new public item without a doc comment will now trigger a compiler warning.

Closed the remaining gaps:

• doc comments on all 9 pub mod declarations in lib.rs
• /// on both encryption helper functions (were // comments)
• /// on GiftWrapMode enum (was // comment)
• /// on missing correlation store methods (new, contains, clear)
• /// on all 4 SessionSnapshot fields
• visual section separators on re-export groups in lib.rs

RUSTFLAGS='-W missing_docs' cargo check --all-features - 0 warnings.