merlint: exclude space-wire from E900, refine E900 to check Codec.v
space-wire has its own C generation approach (lib/wire/Space_wire_3d).
E900 now checks for Wire.Codec.v (codec definition) not just Wire.Codec
(codec usage), reducing false positives on consumer-only packages.
Remaining E900: ocaml-proximity1 (variable-size codec, needs Wire
EverParse support for Field.ref-dependent byte_array sizes).