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 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

• 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 {}.

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

Protected-main reconciliation

Protected main is exact 0841d2ab3d8b5e60a03c0a8e818cf438e2716829. Current exact contributor head is eac07a7125e541c4912b195a7522c319cb2b8792. It is a non-destructive two-parent reconciliation that preserves current protected-main source and overlays only this PR's evidence-error contract plus its focused regression and one code-current changelog edit. The semantic diff remains exactly three files.

No predecessor-head checks or reviews transfer to this moved head.

Exact-current verification

On unchanged exact head eac07a7125e541c4912b195a7522c319cb2b8792:

• CI run 31978497609: success, including Rust contracts and exact owned-production function/line/region/branch coverage;
• SAST Semgrep run 31978497547: success;
• Security Scan run 31978497543: success;
• no inline review threads are currently returned.

The earlier unassigned-runner scanner state was routed with exact identities to existing organization control-plane issue ContextualWisdomLab/.github#712. Both scanner runs subsequently completed successfully without leaf branch churn, so that queue state is retained only as historical transient evidence rather than a source defect or current blocker.

No predecessor-head, queued, skipped, cancelled, absent, stale, status-only, model-only, or synthetic evidence is promoted to current proof.

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 repository governance and every applicable exact-current-head workflow remain authoritative. A qualifying independent current-head/latest-push approval is still required by the live organization ruleset. Protected-main AGENTS.md independently forbids this scheduled actor from merging, self-approving, altering workflows, adding secrets, weakening checks, tagging, or publishing.

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Configuration used: Organization UI

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Run ID: 43c203a5-87ec-4c60-8dbb-756301d92400

📥 Commits

Reviewing files that changed from the base of the PR and between 0841d2a and eac07a7.

📒 Files selected for processing (3)

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

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