refactor: Migrate to C++ fixed-width types and std:: namespace

[ianna]

Summary

This PR updates our type mapping dictionary to use standard C++ type aliases.

Changes

• Migrated legacy C-style types to C++ fixed-width types (e.g., int64_t → std::int64_t).
• Switched from <stdint.h> logic to <cstdint> for better namespace safety.
• Introduced std::size_t for architecture-aware indexing.

Why?

• Portability: Ensures consistent bit-widths across different compilers/architectures.
• Modern Standards: Aligns the codebase with modern C++ practices and aligns with the types ROOT uses in RDataFrame.
• Safety: Using the std:: namespace prevents potential name collisions with 3rd-party libraries.

Fixes #3885
Needs root-project/root#22234

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.43%. Comparing base (95b1cb2) to head (332c925).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/_connect/rdataframe/from_rdataframe.py	97.14% <ø> (ø)