cvc-cli: absorb remaining VolUtils capabilities (from closed volrover#4 salvage audit)

[transfix]

Backlog captured from the salvage audit of volrover #4 (unified volutils CLI), which was closed as superseded by the cvc CLI. The cvc CLI already covers all 19 core volutils subcommands as a strict superset; these are the remaining items the audit flagged as worth preserving so nothing is lost.

A. Wrap existing libcvc capabilities in cvc-cli (logic already in libcvc, just un-exposed)

• Filters: bilateral, contrast-enhancement, anisotropic-diffusion, GDTV (voxels.h) — port the exact legacy VolUtils defaults.
• Arbitrary resize / resample-to-dims — volume::resize.
• Region-restricted stats — compute_stats(vol, region) (currently cvc stats is whole-volume only).
• Trivial op wrappers: difference (vol_difference, volume_ops.h), clamp-min (vol_clamp_min, volume_ops.h), average (N-way fold over vol_average), variable-timestep extract (volume::read(file, var, time)).

B. Preserve genuinely libcvc-absent, novel bits before any future removal of legacy VolUtils from volrover master

• LoG edge — inline 5×5×5 Laplacian-of-Gaussian kernel + zero-padded 3D convolution (no equivalent anywhere in libcvc).
• compare — voxel abs-diff pass/fail with a nonzero exit code on mismatch (CI-checker contract; cvc ssim is similarity-scored, not pass/fail).
• RGBA color-quantization — RawVRGBA2RawIV + its mmap out-of-core backend.
• Cartesian→spherical resampler — cartTospher.
• Segmentation seed generators — criticalPntsToSeed / SeedProduce*.

Source of these lives on volrover master (legacy VolUtils) and in the closed PR branch use-libcvc-volutils. None are blockers; this is an enhancement backlog for cvc-cli.

[transfix]

Progress — Part A + compare landed in #140

Part A (wrap existing libcvc capability) and the self-contained Part B compare are implemented in the cvc CLI and verified end-to-end.

Done (#140):

• Filters: bilateral, contrast, anisotropic, gdtv (legacy VolUtils defaults)
• Arbitrary resize / resample-to-dims — resize --dims X Y Z
• Region-restricted stats — stats --region "x0,y0,z0,x1,y1,z1"
• Trivial op wrappers — difference, clamp-min, average, extract --var/--time
• compare — voxel abs-diff pass/fail with a nonzero exit code (+ --tolerance)

Remaining (follow-up tranche — need translating from volrover's legacy VolMagick-API sources under volrover/src/VolUtils/):

• LoG edge — 5×5×5 Laplacian-of-Gaussian + zero-padded 3D convolution (volEdge.cpp)
• RGBA color-quantization — RawVRGBA2MultiRawIV.cpp + mmap out-of-core backend
• cartesian→spherical resampler — cartTospher.cpp
• segmentation seed generators — criticalPntsToSeed / SeedProduce*