feat: Add IEEE 754 float16 (binary16) support to Rust runtime

[AshharAhmadKhan]

Summary

This PR implements full IEEE 754 half-precision (binary16) float16 support for the Rust runtime as requested in issue #3207.

Implementation Details

Core Type (fory-core/src/float16.rs)

• New type: #[repr(transparent)] pub struct float16(u16) - 614 lines
• IEEE 754 compliant f32 ↔ float16 conversions with proper rounding
• Round-to-nearest, ties-to-even rounding mode
• Complete special value handling: NaN (payload preservation), ±Inf, ±0, subnormals
• Overflow/underflow: Overflow → Infinity, Underflow → Subnormal/Zero
• Policy A comparison: Bitwise Eq/Hash (usable in HashMap) + separate IEEE helpers
• Arithmetic: Implemented via f32 round-back
• Classification methods: is_nan, is_infinite, is_finite, is_normal, is_subnormal, is_zero, is_sign_negative
• Operator traits: Add, Sub, Mul, Div, Neg, PartialOrd
• Display/Debug: Format via to_f32()

Integration

• Buffer methods (buffer.rs): write_f16() and read_f16() using little-endian
• Serializer (serializer/number.rs): Full Serializer trait + ForyDefault implementation
• Type system (types.rs): Added to BASIC_TYPES, PRIMITIVE_TYPES, PRIMITIVE_ARRAY_TYPES, BASIC_TYPE_NAMES, PRIMITIVE_ARRAY_TYPE_MAP, is_primitive_type_id()
• Module export (lib.rs): Public module with Float16 alias

Testing

• ✅ 11 comprehensive unit tests covering all edge cases (ALL PASSING)
• ✅ Full test suite (135 tests, ALL PASSING)
• Test coverage includes:
  • Special values (±0, ±Inf, NaN)
  • Boundary values (max 65504, min normal 2^-14, min subnormal 2^-24)
  • Overflow/underflow behavior
  • Round-to-nearest ties-to-even
  • Arithmetic operations
  • Classification methods
  • Comparison semantics (bitwise & IEEE)

Why?

Support for half-precision floats is essential for:

• Reduced payload size and memory footprint
• ML/graphics interoperability where float16 is common
• Cross-language compatibility with other Fory implementations

What does this PR do?

Adds production-ready IEEE 754 binary16 support to Rust runtime matching the exact specification in #3207, including:

• Strong type safety (no raw u16 in public API)
• Stable Rust only (no nightly features)
• Zero external dependencies
• Complete IEEE 754 compliance
• Comprehensive test coverage

Related issues

Closes #3207

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?

  • Yes: Adds new public type float16 (exported as Float16)
  • New methods: from_bits, to_bits, from_f32, to_f32, classification methods, arithmetic methods
  • New constant values: ZERO, NEG_ZERO, INFINITY, NEG_INFINITY, NAN, MAX, MIN_POSITIVE, MIN_POSITIVE_SUBNORMAL

• Does this PR introduce any binary protocol compatibility change?

  • Yes: Adds FLOAT16 (TypeId = 16) and FLOAT16_ARRAY (TypeId = 50) to wire format
  • Encoded as 2 bytes (little-endian u16 representing IEEE 754 binary16 bits)
  • Backward compatible: Existing code unaffected, new type opt-in only

Benchmark

Not applicable - this PR adds new functionality without modifying existing code paths. No performance impact on existing f32/f64 usage.

[AshharAhmadKhan]

hey @chaokunyang
I notice this PR now has merge conflicts with main. It appears FLOAT8 and BFLOAT16
support was added to main while I was working on FLOAT16.

Should I:

1. Resolve conflicts by merging main and including all three float types?
2. Rebase my branch on latest main?
3. Wait for further direction?

Happy to resolve this whichever way you prefer.

[chaokunyang]

Hi @AshharAhmadKhan , I suggest to merge git main branch and only add support for FLOAT16 in this PR

[AshharAhmadKhan]

Hi @chaokunyang,
I’ve merged the latest main and resolved conflicts, keeping only the FLOAT16 changes in this PR. All tests are passing.
Happy to make any further adjustments if needed.

[chaokunyang]

@AshharAhmadKhan Rust ci still failed, could you fix it first?

[AshharAhmadKhan]

Hi @chaokunyang,

I've resolved all the Clippy warnings and formatting issues.

All 56 CI checks are now passing ✅, including the previously failing Rust CI checks on macOS and Ubuntu.

The implementation is ready for review. Please let me know if any additional changes are needed.

Thanks!