Widen specialized int fast paths to full int64 range

This work widens the interpreter’s specialized int fast paths (tier-2 / uop execution) from the compact-only range to the full int64_t range.

It also fixes follow-up correctness issues in the widened specialized paths for non-compact exact PyLongObjects and on 15-bit builds.

Scope:

• widen specialized integer add/subtract/multiply fast paths to operate across the full int64_t range
• accept exact int operands that fit in int64_t, including non-compact PyLongObjects
• keep specialized in-place mutation compact-only and fall back safely for non-compact inputs
• handle widened integer compare without compact-only assumptions
• construct widened arithmetic results with PyLong_FromInt64() so 15-bit builds do not narrow through stwodigits
• add regression coverage for widened operations, non-compact exact ints, boundary cases, and overflow fallback
• add benchmark scripts for measuring widened specialized integer fast-path performance

Linked PRs

• gh-150425

Start by locating the interpreter’s tier-2/uop specialized integer add, subtract, multiply, and compare fast paths, including handling for PyLongObject operands. Review the regression coverage and benchmark scripts added for widened operations, then verify full int64_t boundaries, non-compact exact ints, overflow fallback, in-place mutation behavior, and 15-bit builds.