Time the replicant-ack budget from the fastest node (low priority)

[emelialei88]

🎯 The "Why" (Intent)

The replicant-ack wait was meant to time the other nodes from the fastest ack (the log even says "fastest node"). But it watched nodes one at a time, in list order, so a lagging node listed first got up to 1s of extra slack per pass. Which node got marked incoherent depended on list order.

🛠️ The "What" (Critical Changes)

• The inline first-ack wait looks at every node at once and sleeps until any of them acks. It checks and sleeps under the lock acks are signalled under, so no ack is missed.
• The per-node check is split out of the node wait first, so it can be run without blocking. This is a pure move.
• New test: two replicants lag 1.5s with a 1s timeout. Exactly one must be marked incoherent, whichever node is fast.

[roborivers]

Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**