perf(v7): lock-free atomic compare-and-swap monotonicity for NewV7

[AdamMagued]

Replace the global timeMu mutex in getV7Time() with an atomic Compare-And-Swap (atomic.CompareAndSwapInt64) state loop.

Motivation

Previously, every call to NewV7() acquired a global mutex timeMu.Lock() to serialize timestamp increments and sub-millisecond sequence numbers. Under multi-goroutine concurrent workloads, this causes severe lock contention and thread synchronization overhead, contending with UUIDv1 operations that also share timeMu.

Solution

• Store the 64-bit monotonically advancing state ((milli << 12) + seq) in lastV7time using lock-free atomic CAS.
• If current timestamp falls behind or equals the previous issued state (next <= last), advance next = last + 1 atomically.
• Added TestVersion7ConcurrentMonotonicity with 16 concurrent goroutines producing 80,000 UUIDs to verify strict monotonicity under concurrent contention.
• 100% backward compatible and passes all unit and fuzz tests.

Fixes #226