[F55] Path C investigation: NmxSvc requires SCM-registered OXID for callbacks

Captured OBJREF byte structures from both paths via the .NET probe:
- `--probe-callback-marshal`: DCOM-marshalled, 338 bytes, succeeds
  (when used inside `MxNativeSession.Open` → `CreateRegisteredService`).
- `--probe-register-managed-callback`: hand-rolled, 162 bytes, fails
  with `RegisterEngine2 → 0x800706BA RPC_S_SERVER_UNAVAILABLE`.

The structural diff:
- `std_flags`: DCOM=`0x0A80` (SORF_OXRES4+6+8) vs hand-rolled=`0x280`
  (SORF_OXRES4+6). Bit `0x0800` (SORF_OXRES8) only set in DCOM.
- ncacn_ip_tcp bindings: DCOM=4 with no ports; hand-rolled=1 with
  explicit `[port]`.
- Total size: 338 vs 162 bytes.

Tested the simplest fix (hand-rolled `std_flags = 0x0A80` to match
DCOM): **still fails with the same 1722.** Reverted.

**Diagnosis updated in F55:** NmxSvc on receiving RegisterEngine2
appears to call `IObjectExporter::ResolveOxid` against the local
SCM (`127.0.0.1:135`) to resolve the callback OBJREF's OXID, then
dial the resulting bindings. Our hand-rolled OXID is never
registered with RPCSS, so the SCM-side resolution fails and NmxSvc
returns RPC_S_SERVER_UNAVAILABLE — matching:
- the symptom (1722),
- the sub-second timing (no TCP dial-back to our listener attempted),
- the fact that the .NET `ManagedCallbackExporter` (same hand-rolled
  approach) ALSO fails identically.

DCOM marshalling fixes this because `CoMarshalInterface` internally
registers the OXID with RPCSS. The bindings have no port because
RPCSS returns the dynamic port from the DCOM stub layer.

**Conclusion: Path A is the architecturally correct fix** — the
callback exporter must be a DCOM-managed object (e.g. via
`windows-rs` `#[implement]`) for NmxSvc to accept the callback.
The hand-rolled-listener-with-explicit-port approach is
fundamentally incompatible with NmxSvc's callback validation, in
both Rust and the .NET reference.

Path C (cheap investigation) is exhausted; F55 verdict updated to
recommend Path A explicitly.

`cargo test --workspace` 824 passing; clippy `-D warnings` clean
across both feature configurations.

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

rust/crates/mxaccess-callback/src/exporter.rs

@@ -210,9 +210,13 @@ impl CallbackExporter {
     /// Build a callback OBJREF to publish back to the AVEVA service.
     ///
     /// Mirrors `ManagedCallbackExporter.CreateCallbackObjRef`
-    /// (`cs:44-54`): the IID is `INmxSvcCallback`, `std_flags = 0x280`,
+    /// (`cs:44-54`): the IID is `INmxSvcCallback`,
     /// `public_refs = 5`, OXID/OID/IPID come from `self.identities`, and
     /// the single string binding is `"<hostname>[<port>]"`.
+    ///
+    /// `std_flags = 0x280` — `SORF_OXRES4 | SORF_OXRES6` (= `0x80 |
+    /// 0x200`). Mirrors the .NET reference's `ManagedCallbackExporter`
+    /// (`cs:48`).
     #[must_use]
     pub fn create_callback_objref(&self, hostname: &str) -> Vec<u8> {
         let binding = format!("{hostname}[{port}]", port = self.local_addr.port());