CultNetMessage::Error gains typed code (CultNetErrorCode) and details
(CultNetErrorDetails) fields mirroring the landed C# reference
(CultNetErrorMessage/CultNetErrorDetails, CultNetSchemaMessages.cs). The
message now encodes/decodes through the raw cultnet.schema.v0 path
(encode_raw_cultnet_schema_message/parse_raw_cultnet_schema_message)
instead of the generic serde_json::Value round trip every other
CultNetMessage variant uses, because that path's BTreeMap-backed
serde_json::Value would sort keys alphabetically and break the C#
reference's fixed schemaVersion/error/routingHint/code/details map
order - every optional key is written nil, never omitted, matching
MessagePack C#'s always-five-key formatter.
Adds the SelectionRefusal -> CultNetMessage mapping the fix-batch-3
comment in selection.rs called out as pending on the C# shape landing,
covering all four codes (selection_invalid, cursor_stale, cursor_invalid,
reference_outside_target).
cultmesh-rs's one CultNetMessage::Error match gets an added `..` so it
still compiles against the wider variant; it does not read code/details.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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