fix(lasercube): stabilize Wi-Fi transport pacing

[SpaceK33z]

Summary

• pace LaserCube Wi-Fi output with a four-packet startup burst, two-packet catch-up limit, and point-rate deadlines
• apply point-rate changes while output is disabled, then wait 50 ms before enabling output
• keep delayed ACK buffer values diagnostic-only and back off persistent UDP send failures
• use a single unicast discovery probe when LASER_DAC_LASERCUBE_IP is set
• add --pps support to the shared frame/stream examples

Validation

• cargo test --features lasercube-network (674 passed, 7 ignored)
• cargo clippy --no-default-features --features lasercube-network --lib --tests
• cargo build --no-default-features --features lasercube-network --example frame --example stream --example reconnect

Hardware test

Tested with a Wi-Fi LaserCube 2.5W running firmware 0.21. A 25-second, 10 kpps circle test completed with zero send errors, about 8 ms ACK latency, and a clean shutdown. The same device remained unreliable at 30 kpps over its current Wi-Fi connection.