The Zodiac killer’s thirteen-symbol cipher (Z13) has resisted formal solution for over half a century. This study does not present a speculative plaintext or guessed name. Instead, it demonstrates that Z13 is a structural remainder of the 340-symbol cipher (Z340) once inherited constraints from the earlier Z408 system exhausted all legal moves.By tracing adjacency, motif, and homophone pressure across the canonical 17×20 Z340 grid, the analysis shows that the cipher’s internal lattice terminates in a closed corridor at Row 14, Columns 10–17 — yielding the stable pattern NANNANLA.The result reframes Z13 not as a miniature cipher but as the physical artifact of constraint collapse.