Make PreallocatedBuffersPerPool runtime-configurable

[lixmal]

Makes WireGuard per-Device memory knobs runtime-configurable for embedded scenarios that create many Devices in one process.

• Convert PreallocatedBuffersPerPool from const to var on all platforms. Defaults preserved: 0 on default/windows, 4096 on android, 1024 on ios.
• Add SetPreallocatedBuffersPerPool(n uint32) to set the default for newly-created Devices.
• Add (*Device).SetPreallocatedBuffersPerPool(n uint32) and (*WaitPool).SetMax(n uint32) to retune live Devices in place. Waiters are broadcast so they re-check against the new cap.
• Add MaxBatchSizeOverride global and (*Device).SetMaxBatchSize(n int) to control the per-Device batch size used by RoutineReceiveIncoming and RoutineReadFromTUN. Each of those goroutines eagerly allocates batch message buffers for its lifetime, so this knob bounds the steady-state per-Device buffer footprint. Zero means "no override" (Devices fall back to max(bind.BatchSize(), tun.BatchSize())); zero is NOT unlimited.
• Device.BatchSize() now honors the override and is used uniformly on both the receive and TUN-read paths.
• Fast path preserved when a pool is constructed with max == 0: Get/Put skip the lock, and SetMax is a no-op. Upstream default behavior is unchanged for callers that never set these knobs.

[lixmal]

@coderabbitai review