Unit test: ReadBuffer — missing tests for getUint8, getUint64, getInt64, getObjectArray, getPosition, getRemainingBytes

[s2x]

Closes #187

Problem

`ReadBuffer` (225 lines, 18 public methods) has good underflow error coverage but is missing tests for several read methods:

1. `getUint8()` — only tested via underflow, no happy-path test
2. `getUint64()` — only tested via underflow, no happy-path test
3. `getInt64()` — only tested via underflow, no happy-path test
4. `getInt16()` — only tested via underflow, no happy-path test
5. `getInt32()` — only tested via underflow, no happy-path test
6. `getObjectArray(string $class)` — never tested at all
7. `getPosition()` — never tested directly
8. `getRemainingBytes()` — only tested for empty/fully-read, not mid-read
9. `peekUint16()` — only tested via underflow, no happy-path test (peek should not advance position)
10. `skip()` — only tested via underflow, no happy-path test
11. `readBytes()` — only tested via underflow, no happy-path test

Expected tests

```php
public function testGetUint8(): void
{
$buf = new ReadBuffer("\xFF");
$this->assertSame(255, $buf->getUint8());
}

public function testGetUint64(): void
{
$buf = new ReadBuffer(pack('J', 12345678901234));
$this->assertSame(12345678901234, $buf->getUint64());
}

public function testGetInt64Negative(): void
{
$buf = new ReadBuffer(pack('q', -1)); // Note: big-endian
$this->assertSame(-1, $buf->getInt64());
}

public function testPeekUint16DoesNotAdvancePosition(): void
{
$buf = new ReadBuffer("\x00\x01\x00\x02");
$peeked = $buf->peekUint16();
$read = $buf->getUint16();
$this->assertSame($peeked, $read); // Same value — position didn't move
}

public function testGetPositionAdvancesCorrectly(): void
{
$buf = new ReadBuffer("\x00\x01\x00\x02\x00\x03");
$this->assertSame(0, $buf->getPosition());
$buf->getUint16();
$this->assertSame(2, $buf->getPosition());
$buf->getUint32();
$this->assertSame(6, $buf->getPosition());
}

public function testGetObjectArray(): void
{
// Build binary for array of KeyValue objects
// Verify correct deserialization
}
```

Acceptance criteria

• Happy-path tests for all 18 public methods
• `peekUint16` verified to not advance position
• `getPosition()` verified after each read operation
• `getRemainingBytes()` verified mid-read
• `getObjectArray()` tested with a real VO class
• Signed integer tests with negative values
• Large unsigned values (near max uint64)