Add FP8 host-side data types (E4M3/E5M2)

[chhwang]

Add FP8 host-side data types (E4M3/E5M2)

Add float8_e4m3_t and float8_e5m2_t host-side types with float
conversion, arithmetic, comparison operators, and std::numeric_limits
specializations. Register FP8_E4M3 and FP8_E5M2 in the data-type
registry and expose them in the Python bindings.

Ported from PR #226 (noli/fp8) with two correctness fixes:

• E4M3 numeric_limits::lowest() returns −448.0 (0xfe), not +448.0.
• E4M3 has_infinity set to false (OCP E4M3 has no infinity encoding).

NumPy and PyTorch have no stable FP8 dtypes, so np/torch mappings
are set to None in the Python registry.

New files:

• ark/float8.h — templated _float8 struct, E4M3/E5M2 type aliases.
• ark/float8.cpp — float↔FP8 conversion routines.
• ark/float8_test.cpp — 386-line test suite covering both types:
  round-trip, subnormals, ±zero, NaN, ±Inf, overflow saturation,
  arithmetic, all comparison operators, numeric_limits invariants.

Changed files:

• ark/api/data_type.cpp — register FP8_E4M3, FP8_E5M2.
• ark/include/ark/data_type.hpp — declare extern constants.
• python/data_type_py.cpp — pybind11 bindings.
• python/ark/data_type.py — Python-side constants.

+1305/−0 across 7 files.

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.48%. Comparing base (d22a90a) to head (1bf7b2f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
ark/float8.h	93.66%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
+ Coverage   85.00%   85.48%   +0.48%     
==========================================
  Files         127      129       +2     
  Lines        6181     6457     +276     
==========================================
+ Hits         5254     5520     +266     
- Misses        927      937      +10     

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