Device layer: pin the /api/* HTTP wire contract in one shared schema module

[subindevs]

Part of #50 (device-layer refactor). Do this first — no behavior change, unblocks every reader of the /api/* surface.

Problem

The device-layer HTTP contract (/api/microscope/execute, /api/devices/stream,
/api/detect_embryos, /api/status, …) is defined only implicitly inside the diSPIM
plugin and re-parsed by hand in three places:

• gently/hardware/dispim/client.py
• gently/app/device_state_monitor.py
• gently/harness/microscope.py

Client and server agree only by convention, so a field rename silently breaks a
consumer.

Goal

Pin the wire contract in one shared schema/dataclass module (request/response
payloads for each /api/* endpoint, plus the SSE event shapes) that both server and
all clients import, so they agree by construction.

Scope

• New module (e.g. gently/harness/wire.py or core/) with dataclasses for each
  endpoint's request/response + stream event.
• Server (device_layer.py handlers) and the three consumers above serialize/parse
  through it.
• Pure refactor: no endpoint added/removed, no behavior change. diSPIM stays byte-for-byte
  compatible on the wire.

Why first

This is the seam #2 depends on — BaseDeviceLayer should speak the shared contract, not
redefine it.