Sign resilience and correctness fixes

[PrexorJustin]

Summary

• one bad tick in the sign update loop used to kill updates for every sign, permanently, until restart. now it just logs and moves on.
• standing signs never got behind-block placement, only wall signs did. fixed.
• a layout with a typo'd rule name loaded fine and only broke things if it ever became the highest-priority match. now it's rejected at load with a clear error.
• SignStateManager.updateSign() was dead code that could never have worked (cast a Location to a Sign). removed.
• /sign list generated a teleport link for a /sign tp subcommand that didn't exist. added it, gated on the TP permission node that was already defined but never used, and both list views now use it consistently.
• some smaller cleanup along the way: deduped a serializer, dropped dead code, fixed a command running on an untracked coroutine scope, cached tab-complete instead of hitting the controller every keystroke.