[M2] mxaccess-rpc: OXID + RemQI body codecs (wave 2)

Lands M2 wave 2 — two pure-Rust body-codec modules under
crates/mxaccess-rpc, plus a small inline ORPC framing port and a
crate-level type consolidation. Resolves F7+F8 from wave 1.

New modules
- guid.rs (4 tests) — hoisted from objref::Guid; shared by all of
  mxaccess-rpc. Resolves F7.
- error.rs — hoisted RpcError union (ShortRead, UnexpectedPacketType,
  UnknownPacketType, InvalidFragmentLength, TruncatedBindBody,
  InvalidAuthTrailer, MissingAuthValue, Decode). Resolves F8.
- orpc.rs (8 tests) — port of OrpcStructures.cs:1-141. ComVersion,
  OrpcThis (32-byte header), OrpcThat (8-byte header),
  MInterfacePointer (length-prefixed OBJREF), StdObjRef (40 bytes).
- object_exporter.rs (~530 LoC, 20 tests) — port of
  ObjectExporterMessages.cs:1-141. IObjectExporter IID, opnums,
  ResolveOxid request encoder + ResolveOxidResult/Failure parsers.
  Owned-string protocol labels cleaned up via Cow upgrade rather than
  Box::leak (ComDualStringEntry::protocol is now Cow<'static, str>).
- rem_unknown.rs (~340 LoC, 11 tests) — port of RemUnknownMessages.cs.
  IRemUnknown IID, RemQueryInterface request/response, RemQiResult.
  4-byte NDR pad in REMQIRESULT preserved as pad_after_hresult per
  CLAUDE.md unknown-bytes rule.

Test count delta: 277 -> 319 (+42; codec 215 unchanged, mxaccess-rpc
60 -> 102, codec parity 2 unchanged).
Open followups touched: F7 + F8 resolved; F9, F10, F11 added.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/crates/mxaccess-rpc/src/error.rs

@@ -0,0 +1,77 @@
+//! Crate-level RPC error type.
+//!
+//! Hoisted from per-module enums in M2 wave 2 — see `design/followups.md` F8.
+//! Every parser/encoder in `mxaccess-rpc` returns this single shared
+//! [`RpcError`] so consumers can match on one error surface across PDU
+//! decode, OBJREF parse, ORPC `ResolveOxid` body decode, and
+//! `IRemUnknown::RemQueryInterface` response decode.
+//!
+//! Variants here are the union of what M1 wave 1 defined locally in
+//! `pdu.rs` and `objref.rs` (`design/followups.md` F8 source list),
+//! plus a generic [`RpcError::Decode`] for one-off conditions wave 2's
+//! ORPC parsers need (referent-id mismatches, conformant-array max-count
+//! underflow, NDR alignment overrun) without growing the enum further.
+
+use thiserror::Error;
+
+/// Errors raised by any codec under `mxaccess-rpc`.
+#[derive(Debug, Error, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum RpcError {
+    /// Buffer was shorter than required to decode the type.
+    #[error("short read: expected {expected} bytes, got {actual}")]
+    ShortRead { expected: usize, actual: usize },
+
+    /// Packet type byte at offset 2 (`DceRpcPdu.cs:52`) did not match the
+    /// expected `DceRpcPacketType` for the parser invoked.
+    #[error("unexpected packet type {actual}, expected {expected}")]
+    UnexpectedPacketType { expected: u8, actual: u8 },
+
+    /// Packet type byte was not a known [`crate::pdu::PacketType`] value.
+    #[error("unknown packet type byte {0}")]
+    UnknownPacketType(u8),
+
+    /// `header.frag_length` is inconsistent with the supplied buffer or
+    /// `auth_length` (`DceRpcPdu.cs:94,150,188,226,101-104,156-159,195-198`).
+    #[error(
+        "fragment length {frag_length} inconsistent with buffer length {buffer_len} \
+         (auth_length={auth_length})"
+    )]
+    InvalidFragmentLength {
+        frag_length: usize,
+        buffer_len: usize,
+        auth_length: usize,
+    },
+
+    /// A bind PDU's per-context list ran past `frag_length`
+    /// (`DceRpcPdu.cs:237`) or a syntax identifier was truncated
+    /// (`DceRpcPdu.cs:354`).
+    #[error("truncated bind body at offset {offset}; need {need} bytes, frag_length={frag_length}")]
+    TruncatedBindBody {
+        offset: usize,
+        need: usize,
+        frag_length: usize,
+    },
+
+    /// Auth-trailer offset is below the 16-byte header
+    /// (`DceRpcPdu.cs:341-345`).
+    #[error("invalid auth trailer offset {offset}")]
+    InvalidAuthTrailer { offset: usize },
+
+    /// Tried to extract an auth value from a PDU whose `auth_length` is 0
+    /// (`DceRpcPdu.cs:336-339`).
+    #[error("PDU has no auth value")]
+    MissingAuthValue,
+
+    /// Generic decode failure with a position and reason. Used by ORPC
+    /// body decoders for one-off conditions that don't justify a typed
+    /// variant (e.g. NDR conformant-array max-count underflow per
+    /// `ObjectExporterMessages.cs:66-69`, referent-id of zero with no
+    /// trailing status per `:57-61`, NDR alignment overrun, etc.).
+    #[error("decode at offset {offset} ({reason}); buffer len {buffer_len}")]
+    Decode {
+        offset: usize,
+        reason: &'static str,
+        buffer_len: usize,
+    },
+}