fix(replication): add audit failure diagnostics

[dirvine]

Summary

• adds permanent audit failure summary fields for challenged, confirmed, absent, and digest-mismatch key counts
• preserves failure classification through responsibility confirmation so final ERROR logs expose the dominant failure mode
• logs audit failure reason and key-count breakdown without per-key ERROR-level noise

Test Plan

• cargo fmt --all
• cargo test replication::audit -- --nocapture
• cargo clippy --all-features -- -D clippy::panic -D clippy::unwrap_used -D clippy::expect_used

Notes

• No ADR added: this is observability/logging instrumentation only, not a behavioural or architectural change.

[Copilot]

Pull request overview

This PR improves replication audit failure diagnostics by adding an aggregated failure summary to audit evidence and emitting richer (non-per-key) ERROR logs that preserve the dominant failure mode after responsibility confirmation.

Changes:

• Introduces AuditFailureSummary and attaches it to FailureEvidence::AuditFailure.
• Classifies digest verification failures as Absent vs DigestMismatch, computes summary counts, and refines the final AuditFailureReason accordingly.
• Updates audit failure logging to include the reason and key-count breakdown.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/replication/types.rs	Adds AuditFailureSummary and a new summary field on FailureEvidence::AuditFailure.
src/replication/mod.rs	Expands audit failure ERROR logging to include reason and summary counts.
src/replication/audit.rs	Classifies per-key audit failures, computes summary counts, preserves/refines failure reason after responsibility confirmation, and adds unit tests for the new helpers.

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