xchain-documentation v0.19.0

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

The XBRIDGE action and the cross-chain bridge protocol are documented, the bridge activation
is documented as keyed per chain, and the canon publishes the TRAIN_ACTIVATION 0.19.0 row
(testnet BTC 152787) and the per-chain testnet bridge heights this train arms (BTC 152929,
LTC 4887898, DOGE 67902062), with testnet activations documented as arming at the tip read
at the cut. The barrier family's canon constants are published inert: the admission-height
margins, the paired mirror-admission flag-day maps, and the anchor-attest arrival margin
and barrier activation. The settle pass's origin-indexer reads, the four bridge helper
modules and the wallet's cross-chain order form are documented; the tracker's per-network
undo-window table and halted-state fields replace a stale value; the node and explorer
configuration pages gain their new keys. Restructured under the platform code-structure standard.

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