Adds a high-16-bit-prefix landblock residency check used as the teleport
worldReady gate — true once the destination landblock's terrain+cells
have been registered via AddLandblock, regardless of whether the caller
passes a canonical (0xFFFF), cell-resolved, or bare landblock id.
Two TDD tests confirm: false before registration, true after, and
that a cell-resolved id on the same landblock returns true.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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