BUG: Let _get_itk_pixelid map np.int64 to itk.int64_t, instead of itk.SL - #6771

This change maps NumPy int64 arrays to ITK’s fixed-width int64_t alias rather than the platform-dependent long alias. The mapping was exercised under simulated LLP64 and LP64 ABI models: the updated code selects a 64-bit type in both cases, while the previous code selected a 32-bit long under LLP64. No defects were found.

T-Rex validation blocked

A native ITK array-conversion run could not be performed because the local environment is missing the required tools and packages: CMake, Ninja, Pixi, NumPy, and ITK. The focused type-mapping check completed without those dependencies.

The change is safe to merge: it corrects the fixed-width type selected for NumPy int64 on LLP64 platforms while retaining 64-bit selection on LP64 platforms.

The only changed mapping was executed against both affected ABI models and showed the intended corrected behavior with no regression in the LP64 case. No review findings remain.

Files Needing Attention: No files need follow-up attention.

What T-Rex did

• Ran the ITK int64 pixel-ID ABI probe for HEAD^ under LLP64 and LP64, then repeated for worktree under both ABI models to compare results.
• Observed that the parent revision used a 32-bit long for LLP64 while the updated revision switched to a fixed-width 64-bit model for LLP64 and LP64, confirming the intended width change without altering LP64 behavior.
• Tried a native ITK conversion run but could not start due to missing CMake, Ninja, Pixi, NumPy, and ITK in the local environment.
• Ran the exact worktree commands for LLP64 and LP64; both models exited with code 0 and after-state showed compatibility across models.
• Captured a blocker snapshot showing the dependencies and tools unavailable, explaining why the native ITK run could not proceed.

_{Ran code and verified through T-Rex}