Harden integer ranges in parallel BC compression

[Roland Shum (ShumWengSang)]

Summary

Add integer-range validation to the existing parallel BC compression iterator:

• Reject dimensions outside the existing signed coordinate range before rounding.
• Promote the dimensions to uint64_t, calculate the rounded block extents and their product, then check the signed loop-count limit before narrowing.
• Reuse the checked width-block count inside the loop.

This follows the review suggestion to use wider intermediate arithmetic and cast back after checking the result. The dimension guard bounds the additions and product, as well as the later signed pixel-coordinate calculations.

This is a source-only, post-allocation arithmetic-hardening change. It does not move validation ahead of destination allocation, change the serial compression path, or change progress-callback behavior. Parallel inputs outside the helper's existing signed dimension/block-count range return an arithmetic-overflow error.

Validation

• MSVC x64 and native x86 Release builds with OpenMP enabled: passed.
• The revised candidate passed six bounded valid-image cases covering BC1/BC3/BC5 on both architectures, comparing serial/parallel output byte-for-byte; largest source was 4 MiB. Earlier x64 baseline runs passed the same ordinary-image cases.
• A separate arithmetic predicate model passed 16 checks per executable, covering 32-bit and 64-bit dimension types, zero extents, signed-dimension limits, and block-product boundaries.
• All executed tests had enforced 256 MiB process-tree and 90-second limits.
• The repository's explicit CI clang-format configuration and git diff --check pass.

Limits

The arithmetic model is not an end-to-end large-image reproduction. No valid file-input reachability or large-allocation mitigation is claimed. No new maintained test-suite regression is included; the local ordinary-image and arithmetic harnesses are validation evidence only.