[filters] Reject out-of-grid centroid coordinates

[fallenmi]

Summary

• reject component coordinates outside the voxel grid before flattening them
• apply the same bounds contract to the typed and PCLPointCloud2 overloads
• add a deterministic 2x2x2 regression for positive, negative, and mixed-axis aliases

Root cause

getCentroidIndexAt() flattened (i, j, k) into a scalar index and only checked that scalar against leaf_layout_. Different coordinate triples can produce the same flattened index, so an out-of-grid coordinate could return an unrelated occupied voxel instead of -1.

For example, in a fully occupied 2x2x2 grid, (2, 0, 0) is outside the grid but currently aliases the valid scalar index for (0, 1, 0). The new component-wise guard rejects the coordinate before subtraction and flattening.

Validation

• reproduced the bug on current master in both overloads with six out-of-grid aliases
• built test_filters successfully
• focused regression: 100 consecutive passes
• full test_filters binary: 22/22 tests passed
• canonical CTest target filters_filters: 1/1 passed
• clang-format 14 changed-lines check and git diff --check passed

AI assistance disclosure: OpenAI Codex helped investigate and draft this change; I reproduced the bug, reviewed every changed line, and ran the tests listed above.

Fixes #3529.