nvx-rs: share one byte-level EXEC and MAPS decoder between the host and the guest #581

Summary

nvx::protocol decodes the NVXC EXEC and MAPS payloads twice. decode_exec_payload and decode_maps_payload produce the host's UTF-8 types, and decode_guest_exec_payload and decode_guest_maps_payload, which #531 adds, produce the guest's byte strings. Each pair repeats the framing, flags, lengths, and field validation, so a later format or validation change can update one decoder and leave the other behind.

Found in the #531 review. #531 keeps a faithful port of the C agent, so this waits until the #358 series is done.

Proposal

Share the byte-oriented structural parsing, and keep the host's UTF-8 types behind conversions:

• Keep the public host API and its diagnostics, and keep accepting non-UTF-8 guest arguments, paths, and environment entries.
• Make intentional host and guest differences explicit rather than substituting one type for the other. For example, the two inherit_default_env fields differ when no explicit environment is supplied: the host's is true (flags & EXEC_ENVIRONMENT_PRESENT == 0 || flags & EXEC_INHERIT_DEFAULT_ENV != 0), and the guest's follows EXEC_INHERIT_DEFAULT_ENV alone.

#537 and #545 can then build on one parsing implementation rather than another pair of codecs.

Tasks

• Share the structural parsing of EXEC and MAPS between the host and guest decoders, with the host API, its diagnostics, and the guest's acceptance unchanged.
• Check the shared decoder against the C agent's parity fixtures, which nvx-rs: replace the guest C programs with Rust binaries #358 checks in before it removes the C agent. Their expected outcomes must not come from the new decoder.