deployPreviousMappingsIfNeeded

Called early during startup (before Geyser scans its custom_mappings/ folder) to copy the previous run's Geyser mappings into place so that Geyser registers our custom-item identifiers at its own boot. The Bedrock pack ZIP is NOT pre-deployed here — it's served live per-session by the platform's pack provider, so a stale pre-deploy can't poison Geyser's in-memory pack cache for the rest of the session.

Mappings still need this pre-boot copy because Geyser's GeyserDefineCustomItemsEvent is a lifecycle event that fires exactly once at startup — there is no per-session mapping refresh, so changes to the custom-item SET require a server restart to take effect.