fix(replication): add first_failed_key to audit-failure log

[dirvine]

Summary

Adds a first_failed_key=0x<16-hex> field to the audit-failure ERROR line in replication::handle_audit_result. This is a follow-up to #129, which already gave us structured counts but no way to identify the specific chunk that was missing.

The line currently looks like:

Audit failure for <peer>: reason=KeyAbsent, confirmed_failed_keys=1, challenged_keys=1, absent_keys=1, digest_mismatch_keys=0

After this PR:

Audit failure for <peer>: reason=KeyAbsent, confirmed_failed_keys=1, challenged_keys=1, absent_keys=1, digest_mismatch_keys=0, first_failed_key=0x18878f1d2d9e0612

first_failed_key is the high-order 8 bytes of the first confirmed_failed_keys entry, hex-encoded with a 0x prefix. 16 hex chars is short enough to keep the log volume low on the hot path but long enough to disambiguate distinct chunks inside the same close group.

Why

• Top-failing-key dashboard: today, the only way to identify which chunk is missing on a peer is to regex-extract the Audit failure for <hex> prefix from the target peer (which is also its XorName, not the chunk XorName). Adding first_failed_key lets us group audit-failures by chunk and by peer without runtime regex.
• Reproducible cross-host correlation: first_failed_key=0x18878f1d2d9e0612 + challenged_peer=<peer-hex> forms a stable (chunk-prefix, target-peer) pair that an operator can paste into a chat thread and anyone can grep for.
• No behaviour change: the only modification is the format string. Trust-event emission, responsibility confirmation, and bootstrap-claim handling are untouched.

Test plan

Three new unit tests in replication::tests cover the helper:

• first_failed_key_label_truncates_to_16_hex_chars — sets the first 8 bytes to a known pattern and the rest to 0xAA, asserts the label is exactly the 16-hex of the first 8 bytes and the lower bytes are dropped.
• first_failed_key_label_falls_back_when_empty — empty input produces "0x" (no misleading default).
• first_failed_key_label_uses_first_key_only — only the first entry of the list is used.

Verified locally:

$ cargo test --lib replication::
test result: ok. 228 passed; 0 failed; 0 ignored; 0 measured; 285 filtered out

$ cargo clippy --lib --all-features
    Finished `dev` profile [optimized + debuginfo] target(s) in 2.70s
# zero warnings

Sample deployed log (before)

2026-06-07T12:17:37.515658Z  ERROR  ant_node::replication
  Audit failure for 18878f1d2d9e061239fe77159f783cff10af2745bddcd8d05dc3ef9522f8528a: reason=KeyAbsent, confirmed_failed_keys=1, challenged_keys=1, absent_keys=1, digest_mismatch_keys=0

Sample deployed log (after, in 0.13.0)

2026-06-07T12:17:37.515658Z  ERROR  ant_node::replication
  Audit failure for 18878f1d2d9e061239fe77159f783cff10af2745bddcd8d05dc3ef9522f8528a: reason=KeyAbsent, confirmed_failed_keys=1, challenged_keys=1, absent_keys=1, digest_mismatch_keys=0, first_failed_key=0x18878f1d2d9e0612

Out of scope (intentionally)

This PR does not address the underlying issue: a peer that passes responsibility confirmation but has zero of its assigned keys. That's a separate concern — the responsibility check in handle_audit_failure should only return confirmed_failures after a grace window has elapsed since the peer joined the close group, otherwise a fresh-joiner will fail every audit until its first sync completes. That fix needs its own PR; the new log line just makes the issue visible enough to measure and triage.

[Copilot]

Pull request overview

Adds a compact first_failed_key=0x<16-hex> label to the replication audit-failure ERROR log line to enable easier cross-host correlation and grouping by the (truncated) chunk key prefix.

Changes:

• Introduces first_failed_key_label() to hex-encode the first 8 bytes of the first confirmed-failed key (or "0x" when empty).
• Extends the audit-failure error! log line in handle_audit_result to include first_failed_key=....
• Adds unit tests covering truncation behavior, empty input fallback, and “first key only” behavior.

Comments suppressed due to low confidence (1)

src/replication/mod.rs:2601

• first_failed_key is computed unconditionally before the error! call. When the logging feature is disabled, the logging macros compile to no-ops and do not evaluate their arguments (see src/logging.rs), but this precomputed String will still allocate/hex-encode on every audit failure. Inline the first_failed_key_label(...) call in the error! invocation so it’s skipped when logging is compiled out.

                let first_failed_key = first_failed_key_label(confirmed_failed_keys);
                error!(
                    "Audit failure for {challenged_peer}: reason={reason:?}, confirmed_failed_keys={}, challenged_keys={}, absent_keys={}, digest_mismatch_keys={}, first_failed_key={first_failed_key}",
                    confirmed_failed_keys.len(),
                    summary.challenged_keys,
                    summary.absent_keys,
                    summary.digest_mismatch_keys,
                );

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