Add packet-level CSS/LoRa-like synchronization and PER lab

[Lay007]

Build the packet-processing layer on top of Lab 8.20/8.21 from #43.

Scope

• Repeated-upchirp preamble generation.
• Packet-start detection and timing metric.
• Sync word and downchirp handling.
• Coarse and fine CFO estimation/correction.
• Sample-rate offset experiment.
• Payload symbol recovery with BER/SER/PER reporting.
• Missed-detection and false-alarm measurements.

The lab should remain clearly named CSS/LoRa-like until interoperability with a commercial LoRa transceiver is demonstrated.

Required artifacts

• timing/synchronization metric plot;
• CFO estimate and residual error;
• SER/PER versus SNR;
• false-alarm and missed-detection statistics;
• JSON metrics with packet and symbol counts;
• RU/EN documentation and experiment manifest.

Acceptance criteria

• deterministic synthetic packet replay passes in CI;
• PER improves after synchronization/CFO correction;
• at least 1000 packets are evaluated per reference point or the confidence limitation is stated;
• the report distinguishes signal-level CSS from LoRa packet compatibility.