Expand Rustdoc coverage on `propchain-traits` to 100%

[nanaf6203-bit]

Problem Statement

Many structs in contracts/traits/src/*.rs have no semantic description.

Why it matters

Library usability.

Technical Context

Walk every pub item and add /// doc with min one example where useful.

Expected Outcome

cargo doc --no-deps shows no warnings on missing docs.

Acceptance Criteria

1. #![warn(missing_docs)] enabled.
2. README badges: documentation coverage.
3. ~80% coverage metrics reflected.

Implementation Notes

Audit + fill.

Files or modules likely to be affected

contracts/traits/src/*.rs.

Dependencies

None.

Difficulty level

MEDIUM.

Estimated effort

M (~3 days).

[starbitz]

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I would like to apply for this issue..new here but I am active

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[Michealshodipo56]

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I would like to work on this kindly assign me

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[Hunterly01]

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I would love to work on this issue expertly, please assign it to me

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[JamesEjembi]

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i would like to work on this issue

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