Refactor float_to_float16_untyped_slow for clarity and optimization

[rfsaliev]

This pull request refactors the float_to_float16_untyped_slow function in float16.h to improve code clarity and maintainability. The logic is now expressed with clear conditional branches instead of a single complex expression, making it easier to understand and maintain while preserving the original functionality.

Note: Change in PR is inspired by #324 made by @mnorris11

Refactoring and code clarity:

• Replaced the single complex return statement in float_to_float16_untyped_slow with explicit conditional branches for handling saturation, normalization, denormalization, and sign extraction, improving readability and maintainability.
• Refactored code is clear and simple. Modern C++ compilers generate well-optimized assembly for the new code - better than previous. See QuickBench results for benchmark code:
  float_to_float16_bench.cpp

[Copilot]

Pull request overview

This PR refactors svs::float16::detail::float_to_float16_untyped_slow (a float→FP16 conversion helper in the public header include/svs/lib/float16.h) from a single compound expression into explicit conditional branches for saturation, normalized, denormalized, and sign-only cases, while preserving the original thresholds and bit-manipulation behavior (including the bounded shift for denormal handling).

Changes:

• Replaces the prior “boolean-mask * value” expression with early-return branches for the same exponent ranges.
• Makes the denormal path’s shift amount explicit and constrained by control flow (avoiding out-of-range shifts for other exponent values).