xchain-sync v0.18.0

xchain-sync v0.18.0, part of the XChain Platform v0.18.0 train.

The follower carries src/train_activation.js, the byte-identical twin of the indexer's
platform-train activation gate, held equal by the parity suite. A replica row shortfall
that survives consecutive equal-height count sweeps escalates once as a distinct
persistent-gap alert and is published on /status as replica_gaps, so a monitor can
alert on a follower that agrees on every hash while missing replicated rows. Replication
presents SYNC_UPSTREAM_KEY on every REST call and on the WebSocket handshake, separate
from the inbound SYNC_API_KEY that guards this process's own API. The reorg sweeps keep
a market whose side is the chain's native coin instead of deleting it as a dangling
ticker, so a replica no longer diverges from the source after a reorg.

Full notes in CHANGELOG.md. The platform release with every component tarball is on xchain-node:
https://github.com/XChain-Platform/xchain-node/releases/tag/v0.18.0