Tiled/Databroker compatibility for composite parameters

[Bilchreis]

Tiled and Databroker only support flat (non-nested) datatypes, but SECoP
allows arbitrarily nested StructOf/TupleOf parameters. This PR adds
support for decomposing shallow composite parameters (nesting depth <= 1)
into individual flat signals so they can be consumed by tiled/databroker,
instead of failing or silently misrepresenting the data.

Composite parameter decomposition

• New CompositeKind classifier (util.classify_datatype) sorts a
  parameter's datatype into:
  • ATOMIC — scalar or array-of-scalar, unchanged single-Signal behaviour.
  • DECOMPOSABLE — a top-level StructOf/TupleOf with depth <= 1: split
    into one read-only param_name_<member> SignalR per member, plus a
    write-only param_name SignalW if the parameter is settable.
  • UNSUPPORTED — deeper nesting (struct/tuple nested inside a
    struct/tuple/array): no signal is generated. Raises
    IncompatibleSECoPDatatype if the parameter is mandatory for the
    module's interface class, otherwise warns and skips it.
• SECoPBackend gains a MEMBER attribute type for these split signals;
  each fetches the parent's cached wire reading and extracts its own
  field/index, so no extra round-trips are incurred.
• status (SECoP's TupleOf(EnumType, StringType)) is now always
  decomposed into status_0/status_1 instead of exposing a single
  structured-numpy-array signal.
• SECoPMoveableDevice is now generic over its target datatype
  (StandardMovable[SignalDatatypeT]); locate(), movable_logic, and
  set_name() were updated to handle the case where target/value are
  decomposed (write-only target, and a private undecomposed value
  readback signal built solely to satisfy StandardMovable's internal
  plumbing).
• GenNodeCode.py updated to codegen the split member signals, per-member
  enum classes for enum-typed struct/tuple members, and the parametrized
  SECoPMoveableDevice[T] base class.

Other changes

• Warn when a generated device name exceeds tiled's 63-character storage
  limit (warn_on_long_device_names).
• Removed dead code: SECoPReadableDevice.observe_status_change and the
  now-unused bluesky.plan_stubs import.

Tests

• New test_device_naming.py for the long-name warning.
• Extended test_classgen.py, test_dtype.py, test_nested.py,
  test_annotation.py, and test_Node.py to cover composite-parameter
  decomposition, CompositeKind classification, and set_name timing.