Fix HLC monotonicity, SigV4 sig comparison, and coordinator redirect timestamp safety

[Copilot]

Four correctness issues flagged in code review on the S3 adapter + coordinator changes.

HLC monotonicity with caller-provided CommitTS

When a non-zero CommitTS is passed by the caller, ShardedCoordinator.dispatchTxn and txnLogs used it directly without observing it on the HLC. Subsequent clock.Next() calls could then issue timestamps smaller than a previously committed CommitTS, breaking MVCC ordering.

// Before: caller-provided commitTS silently bypassed HLC
if commitTS == 0 {
    commitTS = c.nextTxnTSAfter(startTS)
}

// After: HLC is kept monotonic regardless of timestamp origin
if commitTS == 0 {
    commitTS = c.nextTxnTSAfter(startTS)
} else {
    c.clock.Observe(commitTS)
}

Applied in both dispatchTxn and txnLogs.

SigV4 authorization: compare signature only

Full Authorization header string comparison was brittle — semantically equivalent headers differing in whitespace or parameter ordering would fail. Now extracts and compares just the Signature=<hex> component via a new extractS3Signature helper (with table-driven unit tests).

Coordinator redirect: clear CommitTS when StartTS is absent

In the follower redirect path, a caller-supplied CommitTS with StartTS=0 was forwarded as-is. The leader stamps a fresh StartTS independently but won't override an existing CommitTS in the txn meta, producing CommitTS ≤ StartTS. Fix: zero out CommitTS when StartTS is unset so the leader assigns both consistently.

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