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.