nx_placement_test.nx

nx_placement_test.nx -- smoke for nx_placement.nx. Two test classes: A) Lazy lifecycle: build a synthetic Q8_0 block in a buffer; create lazy NxPlacedTensor; verify storage_bytes==0 + is_live==0; materialize; verify is_live==1, storage_bytes>0, values match; demote; verify is_live==0; re-materialize; verify values match. B) Owned: wrap a pre-built NxTensor; verify is_live==1 + storage reflects it; demote refuses (NX_PT_ERR_BAD_PLACEMENT).