**F12 partial improvement** (`mxaccess-rpc::IUnknownHolder` + `mxaccess-nmx`):
- New `IUnknownHolder` newtype that owns an MTA-resident COM proxy
with `unsafe impl Send + Sync`. Mirrors the .NET reference's
`ManagedNmxService2Client._activatedComObject` private field
(`cs:15`).
- New `activate_and_marshal_iunknown_objref(prog_id, ctx)` returns
`(Vec<u8>, IUnknownHolder)`. Existing
`marshal_activated_iunknown_objref` retained as a wrapper that
drops the holder (kept for inline-use callers).
- `NmxClient` gains an `activated_com_object: Option<IUnknownHolder>`
field, populated by `Self::create` from the new helper.
`Self::connect` / `Self::from_bound_transport` set it `None` (no
COM activation in those paths).
- Holding the IUnknown for the client's lifetime keeps the
SCM-tracked OXID valid; without it the COM ref count drops to
zero and the SCM may release the activated server-side instance,
making subsequent `ResolveOxid` / `RemQueryInterface` calls
return `RPC_S_SERVER_UNAVAILABLE`.
**F55 (new) — hand-rolled callback exporter rejected by RegisterEngine2**
Five-step instrumentation of `Session::connect_nmx_auto` proves all
six COM-activation / RemQI / final-bind steps succeed. The 1722
fault originates at `RegisterEngine2` itself:
```
from_nmx_client: callback hostname="DESKTOP-6JL3KKO" port=57886 obj_ref_len=162
from_nmx_client: callback obj_ref hex: 4d454f57010000...
from_nmx_client: RegisterEngine2 (31112, mxaccess.31112)
from_nmx_client: RegisterEngine2 FAIL: Transport(Fault { status: 2147944122 })
```
Status `0x800706BA` = `RPC_S_SERVER_UNAVAILABLE` wrapped as Win32
HRESULT.
**Critical finding: the .NET reference's `--probe-register-managed-callback`
(which uses the same hand-rolled `ManagedCallbackExporter` approach
as the Rust port) ALSO fails with the same `0x800706BA` fault.**
Only `--probe-session-write`, which uses
`ComObjRefProvider.MarshalInterfaceObjRef(callback, ...)` to build
the OBJREF via Windows DCOM proxy/stub marshalling, succeeds. So
this is an architectural artifact of the hand-rolled-callback
design, not a Rust port regression.
`design/followups.md` F55 entry documents the three resolution
paths (switch to DCOM-marshalled callback / hybrid / continue
investigating OBJREF rejection at NmxSvc).
F49 stays open with a refined diagnostic — the per-feature live
verification is gated on F55's resolution.
Workspace tests still 824 passing; clippy `-D warnings` clean
across both feature configurations.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
c5d611d6fa