#2 test: add unit tests for the QR Beam codec

[Anonymous]

Originally created by: Caqil

lib/features/qr_beam/domain/beam_codec.dart (the offline QR-stream codec) has no dedicated Dart tests yet.

Add test/ coverage for:

• encode → decode round-trip across sizes (tiny text, exactly 1 chunk, multi-chunk)
• out-of-order + duplicate frames still reassemble
• corrupted/foreign frames are ignored

The wire format is documented at the top of the file.