Device layer: extract hardware-agnostic BaseDeviceLayer + purge dead RPyC refs

[subindevs]

Part of #50 (device-layer refactor). Depends on the shared wire contract (#79).

Problem

gently/hardware/dispim/device_layer.py is a ~3,000-line monolith mixing
hardware-agnostic plumbing with diSPIM-specific device code. A second hardware plugin
would have to reimplement the plumbing.

Goal

Hoist the reusable scaffolding into a hardware-agnostic BaseDeviceLayer (in
core/harness, building on core/service.py:Service). diSPIM subclasses it and
supplies only device/plan specifics. Pure refactor — behavior identical, so it can
be validated on the real diSPIM before anything new is built on top.

Generic — hoists up

Based on the current routes/machinery in device_layer.py:

• aiohttp routing / Service lifecycle wiring
• SSE device-state streaming (position/property pollers + broadcast/debounce):
  /api/devices/state, /api/devices/stream, /api/devices/callbacks/stream
• plan queue + _plan_executor: /api/queue/item/add, /api/history, /api/plans,
  /api/status
• /api/microscope + /api/microscope/execute plan-dispatch translation
• SAM / detection endpoint scaffolding

Stays in the diSPIM plugin

MMCore init, Ophyd device definitions, Bluesky plans, dual-view/galvo specifics, the
concrete SAM detector, and accessory endpoints (/api/led, /api/temperature,
/api/room_light, /api/camera/*, /api/light_source/*, /api/bottom_camera/stream).

Also in this issue (#50 item 4): purge dead RPyC references

Stale comments/handling from the old RPyC transfer path:

• gently/hardware/dispim/devices/acquisition.py:216
• gently/hardware/dispim/plans/calibration.py:524

Acceptance

• diSPIM boots on BaseDeviceLayer with no behavioral change; @pskeshu validates on-scope.
• The AO-adiSPIM offline profile (#PROFILE) can boot against BaseDeviceLayer without
  importing anything diSPIM-specific — that's the real test that the seam is in the right
  place.

Scope guard (from #50)

Extract only the obviously generic plumbing. Defer a "universal" device-layer
abstraction until a real second hardware validates the seams (rule of three).