xchain-sync v0.19.0

xchain-sync v0.19.0, part of the XChain Platform v0.19.0 train.

The follower halts at a platform-train boundary whose rule set this build lacks, before
writing the boundary block, and announces the pending verdict on /status; with this train
that boundary is testnet BTC 152787, carried in the TRAIN_ACTIVATION 0.19.0 row held
byte-identical to the indexer's registry part, with the identity pin re-derived to match.
The bridge lifecycle table and its rollback twin are mirrored and the replicated schema
version moves to indexer schema 11. Both replica key rebuilds now add the columns they name
so one startup finishes them, and a lookup-table repair retires a superseded local row whose
natural key blocked the source's row. Restructured under the platform code-structure
standard; consensus identity byte-identical and pinned.

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/v0.19.0