Avoid diff allocation for integer range indices

[KookiesNKareem]

Summary

This PR adds range-specific fast paths for integer OrdinalRange indices when Reactant checks slice stride and index contiguity.

The existing generic fallback calls diff(x), which materializes an O(n) vector just to inspect the stride/contiguity of ranges such as 1:n, 1:2:n, and colon-derived Base.Slice(Base.OneTo(n)). Integer ordinal ranges already expose this information through step(x), so these checks can be O(1) and allocation-free.

The change is deliberately narrow:

• applies only to OrdinalRange{<:Integer,<:Integer}
• preserves empty range behavior for get_slice_stride(1:0) == -1
• preserves single-element behavior for get_slice_stride(1:1) == 1
• covers colon-derived Base.Slice(Base.OneTo(n)) indices
• leaves vector, boolean, traced, non-integer, and custom non-ordinal index paths on the existing fallback

Benchmark evidence

Measured on NVIDIA TITAN RTX with Reactant loaded:

get_slice_stride(1:1_000_000)
before: 5621.363 us, 8,000,072 bytes
after:     0.124 us, 0 bytes

__contiguous_indices(1:1_000_000)
before: 5650.619 us, 8,000,072 bytes
after:     0.024 us, 0 bytes

collect(1:100000) vector inputs remain on the existing fallback path:
~539 us / ~528 us, 800,072 bytes

Validation

Added focused regression coverage in test/core/indexing.jl for:

• empty integer ranges preserving the existing no-slice sentinel
• contiguous integer ranges reporting stride 1 and contiguity
• strided integer ranges reporting their explicit step and non-contiguity
• colon-derived Base.Slice(Base.OneTo(n)) ranges using the fast path

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.44%. Comparing base (b39a1fc) to head (6832236).
⚠️ Report is 1143 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3014       +/-   ##
===========================================
- Coverage   68.16%   43.44%   -24.73%     
===========================================
  Files         109      223      +114     
  Lines       11779    31598    +19819     
===========================================
+ Hits         8029    13727     +5698     
- Misses       3750    17871    +14121     

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