[JavaScript] add bfloat16 and bfloat16_array

[chaokunyang]

Feature Request

Add bfloat16 and bfloat16_array support to Fory JavaScript runtime and codegen, following the same structure as the existing float16 work.

Related float16 issue: #3210 (open)

Is your feature request related to a problem? Please describe

We want to use bfloat16 (BF16) in FDL to reduce payload size while keeping a wide exponent range (common in ML/AI workflows). Fory currently lacks a BF16 primitive and optimized arrays.

Describe the solution you'd like

1) FDL / Type System

• Introduce a new primitive type: bfloat16.
• Add bfloat16_array as a packed array type (or treat it as a canonical optimized form of repeated bfloat16).
• Allow bfloat16 in message fields, repeated fields, map values, and unions (where primitives are allowed).

2) Wire Format / Serialization Semantics

• Encode bfloat16 as 2 bytes representing the raw IEEE 754 bfloat16 bit pattern.
• Endianness must match existing float32/float64 behavior.
• NaN/Inf/±0/subnormal must round-trip correctly at the bit level (document NaN policy if canonicalized).

3) JavaScript Runtime (core requirement)

Provide a public strong type and conversions, not raw uint16 in public APIs.

3.1 Type definition

• Type: BFloat16
• Suggested representation: class BFloat16 { constructor(bits) { /* uint16 */ } }
• Provide to_bits() / from_bits() equivalents.

3.2 Conversions (IEEE 754 compliant)

• Convert float32 -> bfloat16 with round-to-nearest, ties-to-even.
• Convert bfloat16 -> float32 exactly.
• Correct handling of NaN/Inf/±0/subnormals.

3.3 Arrays / bfloat16_array

• Provide BFloat16Array as a packed contiguous array of u16 bits.
• Suggested storage: BFloat16Array backed by Uint16Array (packed u16 bits)
• Mapping: bfloat16_array -> Uint16Array (or BFloat16Array wrapper), repeated bfloat16 -> BFloat16[] with fast-path for Uint16Array

3.4 Serialization boundary

• Serializer/deserializer should read/write the underlying 16-bit pattern.
• Public APIs should not expose raw uint16 for values.

4) Codegen requirement (JavaScript)

• Generated fields for bfloat16 must use BFloat16.
• Generated fields for bfloat16_array must use BFloat16Array or the packed container noted above.

5) Compiler/Reflection integration

• Update type resolver/reflection to treat BFloat16 as the bfloat16 primitive (distinct from uint16).

6) Tests

• Scalar conversions (±0, ±Inf, NaN, subnormals, max/min normal).
• Array serialization tests for packed bfloat16_array.
• Round-trip tests inside messages (including repeated/map use).

Describe alternatives you've considered

1. Store BF16 as float32 in memory and downcast only on serialization.

• Rejected: increases memory footprint and defers rounding, causing cross-language inconsistencies.

2. Expose raw uint16 in public APIs.

• Rejected: error-prone and inconsistent with strong-typed primitives.

Additional context

This issue mirrors the structure of the float16 work but targets bfloat16 + bfloat16_array.

[miantalha45]

Hi @chaokunyang
Please assign this issue to me, i want to make this enhancement.

[chaokunyang]

@miantalha45 Assigned to you

[miantalha45]

@chaokunyang I have raised a PR to fix this. Please check.