Add vertical neighbor meshing verification

[NearDrive]

Motivation

• Ensure the chunk mesher properly culls faces against vertical neighbors (chunks above/below) to avoid vertical "holes" at chunk boundaries.
• Provide an automated check that will catch regressions where the mesher treats a loaded vertical neighbor incorrectly or fails to treat unloaded neighbors as kBlockAir.

Description

• Add CheckMesherVerticalNeighbors in src/core/Verify.cpp which creates a base chunk and an above chunk, sets blocks at the vertical boundary, calls ChunkMesher::BuildMesh, and asserts the resulting vertex and index counts hide the shared +Y face.
• Wire CheckMesherVerticalNeighbors into RunAll so the new check runs as part of the existing verify suite.
• The test expects one face to be culled (expecting 5 faces total for the test case) and checks both mesh.vertices.size() and mesh.indices.size() for correctness.

Testing

• No automated tests were executed as part of this change; the new check is added to the core::Verify suite and will run when the verify suite is invoked.
• The change is limited to adding a verification check and does not modify the mesher implementation itself, so compilation/run validation should be performed by the project's normal build/test steps.

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Codex Task