zig_ethp2p: RLNC and additional EC Scheme types

[ch4r10t33r]

Reed–Solomon path matches current ethp2p RS usage. RLNC and other Scheme types from the reference stack are out of tree until a concrete protocol need appears.

[ch4r10t33r]

Started work: src/layer/ec_scheme.zig adds EcSchemeKind (reed_solomon | rlnc), wireName() ("reed-solomon" matches Go NewScheme), and isImplemented() (only RS today). Exported as layer.ec_scheme; README + UPSTREAM updated. Next steps for this issue: RLNC preamble/chunk wire layout per spec, GF(2^8) coding path, and strategy integration (or a separate RLNC module + channel wiring).