chore(dart): refine dart xlang serialization (#3596)

## Why?



## What does this PR do?

- comprehensive tests
- align time serializers
- refactor fixed int support
- add bfloat16/float16 support


## Related issues

#1194 
#1017 

## AI Contribution Checklist



- [ ] Substantial AI assistance was used in this PR: `yes` / `no`
- [ ] If `yes`, I included a completed [AI Contribution
Checklist](https://github.com/apache/fory/blob/main/AI_POLICY.md#9-contributor-checklist-for-ai-assisted-prs)
in this PR description and the required `AI Usage Disclosure`.
- [ ] If `yes`, my PR description includes the required `ai_review`
summary and screenshot evidence of the final clean AI review results
from both fresh reviewers on the current PR diff or current HEAD after
the latest code changes.



## Does this PR introduce any user-facing change?



- [ ] Does this PR introduce any public API change?
- [ ] Does this PR introduce any binary protocol compatibility change?

## Benchmark

Author: chaokunyang

dart/packages/fory-test/lib/entity/xlang_test_manual.dart

@@ -70,17 +70,13 @@ final class _Union2Serializer extends UnionSerializer<Union2> {
   const _Union2Serializer();
 
   @override
-  void write(WriteContext context, Union2 value) {
-    final buffer = context.buffer;
-    buffer.writeVarUint32(value.index);
-    context.writeRef(value.value);
-  }
+  int caseId(Union2 value) => value.index;
+
+  @override
+  Object caseValue(Union2 value) => value.value;
 
   @override
-  Union2 read(ReadContext context) {
-    final buffer = context.buffer;
-    final index = buffer.readVarUint32();
-    final value = context.readRef();
+  Union2 buildValue(int index, Object? value) {
     if (index == 0 && value is String) {
       return Union2.ofString(value);
     }

dart/packages/fory-test/lib/entity/xlang_test_models.dart

@@ -440,14 +440,14 @@ class CircularRefStruct {
 class UnsignedSchemaConsistent {
   UnsignedSchemaConsistent();
 
-  UInt8 u8Field = UInt8(0);
-  UInt16 u16Field = UInt16(0);
+  Uint8 u8Field = Uint8(0);
+  Uint16 u16Field = Uint16(0);
 
   @Uint32Type(compress: true)
-  UInt32 u32VarField = UInt32(0);
+  Uint32 u32VarField = Uint32(0);
 
   @Uint32Type(compress: false)
-  UInt32 u32FixedField = UInt32(0);
+  Uint32 u32FixedField = Uint32(0);
 
   @Uint64Type(encoding: LongEncoding.varint)
   int u64VarField = 0;
@@ -459,18 +459,18 @@ class UnsignedSchemaConsistent {
   int u64TaggedField = 0;
 
   @ForyField(nullable: true)
-  UInt8? u8NullableField;
+  Uint8? u8NullableField;
 
   @ForyField(nullable: true)
-  UInt16? u16NullableField;
+  Uint16? u16NullableField;
 
   @ForyField(nullable: true)
   @Uint32Type(compress: true)
-  UInt32? u32VarNullableField;
+  Uint32? u32VarNullableField;
 
   @ForyField(nullable: true)
   @Uint32Type(compress: false)
-  UInt32? u32FixedNullableField;
+  Uint32? u32FixedNullableField;
 
   @ForyField(nullable: true)
   @Uint64Type(encoding: LongEncoding.varint)
@@ -502,18 +502,18 @@ class UnsignedSchemaCompatible {
   UnsignedSchemaCompatible();
 
   @ForyField(nullable: true)
-  UInt8? u8Field1;
+  Uint8? u8Field1;
 
   @ForyField(nullable: true)
-  UInt16? u16Field1;
+  Uint16? u16Field1;
 
   @ForyField(nullable: true)
   @Uint32Type(compress: true)
-  UInt32? u32VarField1;
+  Uint32? u32VarField1;
 
   @ForyField(nullable: true)
   @Uint32Type(compress: false)
-  UInt32? u32FixedField1;
+  Uint32? u32FixedField1;
 
   @ForyField(nullable: true)
   @Uint64Type(encoding: LongEncoding.varint)
@@ -527,14 +527,14 @@ class UnsignedSchemaCompatible {
   @Uint64Type(encoding: LongEncoding.tagged)
   int? u64TaggedField1;
 
-  UInt8 u8Field2 = UInt8(0);
-  UInt16 u16Field2 = UInt16(0);
+  Uint8 u8Field2 = Uint8(0);
+  Uint16 u16Field2 = Uint16(0);
 
   @Uint32Type(compress: true)
-  UInt32 u32VarField2 = UInt32(0);
+  Uint32 u32VarField2 = Uint32(0);
 
   @Uint32Type(compress: false)
-  UInt32 u32FixedField2 = UInt32(0);
+  Uint32 u32FixedField2 = Uint32(0);
 
   @Uint64Type(encoding: LongEncoding.varint)
   int u64VarField2 = 0;

dart/packages/fory/README.md

@@ -13,8 +13,9 @@ cases.
 - Compatible mode for schema evolution
 - Optional reference tracking for shared and circular object graphs
 - Manual serializers for external types, custom payloads, and unions
-- Explicit xlang value wrappers such as `Int32`, `UInt32`, `Float16`,
-  `Float32`, `LocalDate`, and `Timestamp`
+- Explicit xlang value wrappers such as `Int32`, `Uint32`, `Float16`,
+  `Bfloat16`, `Float32`, `LocalDate`, and `Timestamp`, plus `Duration`
+  support
 
 ## Getting Started
 
@@ -219,37 +220,47 @@ void main() {
 
 ## Type Mapping
 
-Dart has no native fixed-width 8/16/32-bit integer or single-precision float
-types. Fory Dart provides thin wrapper types (`Int8`, `Int16`, `Int32`, `UInt8`,
-`UInt16`, `UInt32`, `Float16`, `Float32`) imported from `package:fory/fory.dart`
-to represent these xlang wire types.
-
-| Fory xlang type | Dart type                |
-| --------------- | ------------------------ |
-| bool            | `bool`                   |
-| int8            | `fory.Int8` (wrapper)    |
-| int16           | `fory.Int16` (wrapper)   |
-| int32           | `fory.Int32` (wrapper)   |
-| int64           | `int`                    |
-| float16         | `fory.Float16` (wrapper) |
-| float32         | `fory.Float32` (wrapper) |
-| float64         | `double`                 |
-| string          | `String`                 |
-| binary          | `Uint8List`              |
-| local_date      | `LocalDate`              |
-| timestamp       | `Timestamp`              |
-| list            | `List`                   |
-| set             | `Set`                    |
-| map             | `Map`                    |
-| enum            | `enum`                   |
-| named_struct    | `class`                  |
-| bool_array      | `List<bool>`             |
-| int8_array      | `Int8List`               |
-| int16_array     | `Int16List`              |
-| int32_array     | `Int32List`              |
-| int64_array     | `Int64List`              |
-| float32_array   | `Float32List`            |
-| float64_array   | `Float64List`            |
+Dart has no native fixed-width 8/16/32-bit integer, unsigned 64-bit integer,
+or single-precision float types. Fory Dart provides thin wrapper types
+(`Int8`, `Int16`, `Int32`, `Uint8`, `Uint16`, `Uint32`, `Uint64`, `Float16`,
+`Bfloat16`, `Float32`) imported from `package:fory/fory.dart` to represent
+these xlang wire types. For 16-bit floating-point arrays, Dart exposes
+`Float16List` and `Bfloat16List` as contiguous fixed-length buffers.
+
+| Fory xlang type | Dart type                 |
+| --------------- | ------------------------- |
+| bool            | `bool`                    |
+| int8            | `fory.Int8` (wrapper)     |
+| int16           | `fory.Int16` (wrapper)    |
+| int32           | `fory.Int32` (wrapper)    |
+| int64           | `int`                     |
+| uint8           | `fory.Uint8` (wrapper)    |
+| uint16          | `fory.Uint16` (wrapper)   |
+| uint32          | `fory.Uint32` (wrapper)   |
+| uint64          | `fory.Uint64` (wrapper)   |
+| float16         | `fory.Float16` (wrapper)  |
+| bfloat16        | `fory.Bfloat16` (wrapper) |
+| float32         | `fory.Float32` (wrapper)  |
+| float64         | `double`                  |
+| string          | `String`                  |
+| binary          | `Uint8List`               |
+| duration        | `Duration`                |
+| local_date      | `LocalDate`               |
+| timestamp       | `Timestamp`               |
+| list            | `List`                    |
+| set             | `Set`                     |
+| map             | `Map`                     |
+| enum            | `enum`                    |
+| named_struct    | `class`                   |
+| bool_array      | `List<bool>`              |
+| int8_array      | `Int8List`                |
+| int16_array     | `Int16List`               |
+| int32_array     | `Int32List`               |
+| int64_array     | `Int64List`               |
+| float16_array   | `Float16List`             |
+| bfloat16_array  | `Bfloat16List`            |
+| float32_array   | `Float32List`             |
+| float64_array   | `Float64List`             |
 
 ## Public API
 
@@ -265,9 +276,9 @@ The main exported API includes:
   annotations
 - `Int32Type`, `Int64Type`, `Uint32Type`, `Uint64Type` — numeric encoding
   overrides
-- Numeric wrappers: `Int8`, `Int16`, `Int32`, `UInt8`, `UInt16`, `UInt32`,
-  `Float16`, `Float32`
-- Temporal wrappers: `LocalDate`, `Timestamp`
+- Numeric wrappers: `Int8`, `Int16`, `Int32`, `Uint8`, `Uint16`, `Uint32`,
+  `Uint64`, `Float16`, `Bfloat16`, `Float32`
+- Temporal types: `LocalDate`, `Timestamp`, `Duration`
 
 ## Cross-Language Notes
 

dart/packages/fory/lib/fory.dart

@@ -49,8 +49,15 @@ export 'src/meta/type_ids.dart';
 export 'src/serializer/enum_serializer.dart';
 export 'src/serializer/serializer.dart';
 export 'src/serializer/union_serializer.dart';
-export 'src/types/fixed_ints.dart';
+export 'src/types/bfloat16.dart';
 export 'src/types/float16.dart';
 export 'src/types/float32.dart';
+export 'src/types/int16.dart';
+export 'src/types/int32.dart';
+export 'src/types/int8.dart';
 export 'src/types/local_date.dart';
 export 'src/types/timestamp.dart';
+export 'src/types/uint16.dart';
+export 'src/types/uint32.dart';
+export 'src/types/uint64.dart';
+export 'src/types/uint8.dart';

dart/packages/fory/lib/src/buffer.dart

@@ -22,6 +22,7 @@ import 'dart:typed_data';
 
 import 'package:meta/meta.dart';
 
+import 'package:fory/src/types/bfloat16.dart';
 import 'package:fory/src/types/float16.dart';
 
 final BigInt _mask64Big = (BigInt.one << 64) - BigInt.one;
@@ -255,6 +256,12 @@ final class Buffer {
   /// Reads a half-precision floating-point value.
   Float16 readFloat16() => Float16.fromBits(readUint16());
 
+  /// Writes a bfloat16 floating-point value.
+  void writeBfloat16(Bfloat16 value) => writeUint16(value.toBits());
+
+  /// Reads a bfloat16 floating-point value.
+  Bfloat16 readBfloat16() => Bfloat16.fromBits(readUint16());
+
   /// Writes [value] verbatim.
   void writeBytes(List<int> value) {
     ensureWritable(value.length);
@@ -409,13 +416,12 @@ final class Buffer {
 
   /// Writes a tagged unsigned 64-bit integer.
   void writeTaggedUint64(int value) {
-    final unsigned = _toUnsigned64(value);
-    if (unsigned <= 0x7fffffff) {
-      writeInt32((unsigned << 1) & 0xffffffff);
+    if (value >= 0 && value <= 0x7fffffff) {
+      writeInt32(value << 1);
       return;
     }
     writeUint8(0x01);
-    writeUint64(unsigned);
+    writeUint64(value);
   }
 
   /// Reads an unsigned 64-bit integer written by [writeTaggedUint64].
@@ -487,8 +493,6 @@ Uint8List bufferBytes(Buffer buffer) => buffer._bytes;
 @internal
 ByteData bufferByteData(Buffer buffer) => buffer._view;
 
-int _toUnsigned64(int value) => (BigInt.from(value) & _mask64Big).toInt();
-
 extension on Buffer {
   void _writeVarUint64BigInt(BigInt value) {
     var remaining = value & _mask64Big;