Backport srv03-style SCSIPORT shutdown plumbing into NT 3.5 SCSIPORT

## Context

NT 3.5's SCSIPORT (`src/NT/PRIVATE/NTOS/DD/SCSIPORT/`) has zero shutdown
notification machinery for miniports — no `IoRegisterShutdownNotification`,
no `IRP_MJ_SHUTDOWN` dispatch, no `HwAdapterControl(ScsiStopAdapter)`
plumbing. Miniports that implement `HwAdapterControl` (e.g. nvme2k) wire
the entry point but it is *never invoked* on this kernel.

## Symptom we already mitigated

`nvme2k` reported `ERROR - Got NULL SRB for CID=N` deterministically at
kernel halt. Root cause is the latent race: NVMe controller continues
to post CQ entries while the FS layer has given up on those tags, the
miniport's FallbackTimer fires, `ScsiPortGetSrb` returns NULL.

Quick fix in 934188d (this tree) and techomancer/nvme2k#28 (upstream):
demote the "NULL SRB" log to an `_EXTRA` trace — the driver already
handles NULL gracefully (consume CQ, advance head). Resolves the visible
symptom on a kernel without shutdown notification.

## What the proper backport would add

Port the shutdown core from W2003-vintage SCSIPORT
(`drivers/storage/scsiport/{enable.c,control.c}`):

- `SpShutdownAdapter` — sets `PD_SHUTDOWN_IN_PROGRESS`, cancels the
per-adapter miniport timer (`KeCancelTimer`), runs
`SpShutdownAdapterSynchronized` under ISR sync.
- `SpShutdownAdapterSynchronized` — calls
`HwAdapterControl(ScsiStopAdapter, NULL)`, **then** masks IRQs.
(Ordering matches NVMe 1.4c §7.6.2: controller stays interrupt-live
through the shutdown notification.)
- `SpCallAdapterControl` + `SpIsAdapterControlTypeSupported` +
`SpGetSupportedAdapterControlFunctions` — dispatch helpers.

W2003 triggers this from PnP/Power callbacks, which NT 3.5 doesn't have.
We'd wire from the NT-3.5-appropriate path instead:

- `IoRegisterShutdownNotification(adapterDevObj)` per adapter at init.
- `MajorFunction[IRP_MJ_SHUTDOWN]` dispatch entry → calls
`SpShutdownAdapter`.

## Cost estimate

~120 LOC ported + ~20 LOC of init/dispatch glue. Mostly in
`SCSIPORT/PORT.C` and `INTERNAL.C`. Half a day.

## What we'd gain

- Architectural correctness — devices get to quiesce gracefully before
HAL hands back to firmware.
- `NvmeShutdownController` (body completed in 934188d but currently
dormant) becomes live: NVMe 1.4c §7.6.2 sequence actually runs.
- The `ShuttingDown` flag plumbed through 934188d becomes load-bearing
(currently set/unset but no reader).
- Future hardware-targeting miniports get the hook for free.

## Why this is deferred

For our current target (QEMU TCG NVMe), the device-side cache is
in-memory anyway — there's no data-persistence implication, no
behavioural change. The demote already fixed the visible symptom.

Worth pursuing if/when:
- MicroNT targets real hardware with on-board write caches.
- A new miniport with non-trivial shutdown semantics is added (real
AHCI driver, anything with persistent device-side state).
- A standalone "modernise NT 3.5 SCSIPORT to ~W2K-equivalent" arc is
scoped.