fix(evidence): expose standard evidence error contract

[seonghobae]

Buyer/reliability gap

originweave-evidence::EvidenceError is a public validation failure type used by network and provenance evidence construction, but protected main 0c376acf059be9ddddddfbde1d0189e4f39ef014 does not implement Rust Display / std::error::Error for it. Callers therefore cannot render or chain this stable typed failure through ordinary Rust error boundaries without special-case matching.

Test-first evidence

This PR is based directly on protected main 0c376acf059be9ddddddfbde1d0189e4f39ef014.

• Test-only head 70b900d337be10ee6fa5b2fbbca2c4b9a65b0142 first failed only at cargo fmt --check, so that run was not accepted as RED.
• Formatting-only head b951f6863a4abc1308ebada86299f8a0a341def8 reached the intended compile boundary: the new integration test could not call to_string() or Error::source() because EvidenceError lacked Display and std::error::Error. This is the valid RED.
• Production head f151c24ec4bd6206891236f574d9c1063d9594d0 added only deterministic credential-safe Display text for the five existing variants plus impl std::error::Error for EvidenceError {}. Exact-head Rust contracts and production coverage both passed, including the repository's exact owned production function/line/region/branch coverage gate.
• Current exact head ab90ff5013bf1dfa4002de17816cafe3b86f156a adds the required canonical changelog entry. Exact-current-head CI run 31825085853, SAST Semgrep run 31825085856, and Security Scan run 31825085880 are all completed successfully.

The regression also requires Error::source() to remain empty for every current variant.

Scope boundary

This lane changes no evidence validation rule, redaction behavior, network capture, provenance data, persistence, retention, policy, workflow, or secret handling. The production remedy is limited to the standard Rust error contract for the existing five public variants.

Live review state

This PR is Ready for review and mergeable on exact head ab90ff5013bf1dfa4002de17816cafe3b86f156a. There are currently no formal reviews or unresolved review threads. Passing automation is not independent approval; protected-main governance still requires an eligible independent approval and latest-push approval, and the scheduled actor does not merge or self-approve.

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📥 Commits

Reviewing files that changed from the base of the PR and between 0c376ac and ab90ff5.

📒 Files selected for processing (3)

• CHANGELOG.md
• crates/originweave-evidence/src/lib.rs
• crates/originweave-evidence/tests/error_contract.rs

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