Fix default machine ID to respect BitsMachineID setting

[Copilot]

When BitsMachineID < 16 is specified without a custom MachineID function, the default lower16BitPrivateIP() returns all 16 bits from the IP address, which can exceed 1<<BitsMachineID and trigger ErrInvalidMachineID.

Changes

• v2/sonyflake.go: Mask default machine ID to sf.bitsMachine bits when using IP-based default
• v2/mock/mock.go: Add NewInterfaceAddrsWithIP() helper for testing with arbitrary IPs
• v2/machine_bits_test.go: Validate masking behavior with 8, 10, and 16-bit configurations

Example

// Previously failed with ErrInvalidMachineID when IP's lower 16 bits exceeded 1024
sf, err := New(Settings{
    BitsMachineID: 10,  // Max machine ID = 1024
    // MachineID not provided, uses default IP-based ID
})
// Now succeeds - machine ID masked to lower 10 bits of IP

The fix applies only to v2 where BitsMachineID is configurable. v1 remains unchanged with its fixed 16-bit machine ID.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>The default machine should not always have 16 bits.</issue_title>
<issue_description>So currently, if machineId isn't provided the default is to get the 16 lower bits of the private ip, but in case the user provides BitsMachineId = 10 and not the MachineId in that case also the lib will try to set the machineId to 16 lower bits of the private ip which will raise the ErrInvalidMachineID because of

sf.machine >= 1<<sf.bitsMachine {
		return nil, ErrInvalidMachineID
	}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes The default machine should not always have 16 bits. #82

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