Closes #186

Problem

`WriteBuffer` (159 lines, 15 public methods) has only 14 tests covering basic operations. Missing coverage:

1. `addInt64(int)` — never tested (signed 64-bit integer)
2. `addArray(ToStreamBufferInterface ...)` — never tested (serializes array of objects)
3. `addStringArray(string ...)` — never tested
4. Fluent chaining — no test verifies that chaining multiple `add*()` calls produces correct output
5. Boundary values — no tests for max/min values of each integer type
6. `addUInt32` / `addUInt64` out-of-range — only UInt8 and UInt16 have range validation tests

Expected tests

```php
public function testAddInt64(): void
{
$buf = (new WriteBuffer())->addInt64(-1);
$this->assertSame(str_repeat("\xFF", 8), $buf->getContents());
}

public function testAddInt64MaxValue(): void
{
$buf = (new WriteBuffer())->addInt64(PHP_INT_MAX);
$this->assertSame(pack('J', PHP_INT_MAX), $buf->getContents());
}

public function testAddArray(): void
{
$item1 = new PublishedMessage(0, 'hello');
$item2 = new PublishedMessage(1, 'world');
$buf = (new WriteBuffer())->addArray($item1, $item2);
// Verify: int32 count + item1 binary + item2 binary
}

public function testAddStringArray(): void
{
$buf = (new WriteBuffer())->addStringArray('foo', 'bar');
// Verify: int32 count + string1 + string2
}

public function testFluentChaining(): void
{
$buf = (new WriteBuffer())
->addUInt16(0x0001)
->addUInt16(0x0001)
->addUInt32(1)
->addString('test');
// Verify complete binary output
}

public function testUInt32MaxValue(): void
{
$buf = (new WriteBuffer())->addUInt32(0xFFFFFFFF);
$this->assertSame("\xFF\xFF\xFF\xFF", $buf->getContents());
}
```

Acceptance criteria

• `addInt64` with positive, negative, zero, min, max values
• `addArray` with 0, 1, and multiple items
• `addStringArray` with 0, 1, and multiple strings
• Fluent chaining produces correct concatenated output
• Boundary values for all integer types (0, max, min where applicable)