Forge doesn't split client/server jars, so a modded server's own classpath
already has every loaded mod's block textures, unused server-side.
BlockAssetExtractor reads them via the classloader (best-effort convention
match on registry name -> texture filename) plus the numeric-id -> registry-
name mapping, and ships both to the backend once per connection via two new
wire messages (block_registry, block_textures), fired through a new
BackendConnection#setReadyListener callback so the one-time send can't race
the async WS handshake. Only forge-1_12_2 wires it up so far; the other two
leaves can adopt it later with no protocol change.
octoturge
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