@attr decorator sugar over getter/setter constructors

[coretl]

Adds the @attr decorator sugar over the getter=/setter= constructors from #392 (ADR 0018, 0014 review update).

Scope

• @attr is a decorator only, two forms: bare @attr and parameterised @attr(precision=3, units="V", update_period=0.5) on an async getter, plus property-style @my_attr.setter on the writer.
  • getter only → AttrR; getter + .setter → AttrRW (deferred creation). There is no @attr_r/@attr_rw.
  • No free-function attr() factory — the procedural spelling is AttrR(getter=…) / AttrRW(getter=…, setter=…) (from getter/setter IO rework; remove AttributeIORef and DataType #392); @attr is pure sugar over it.
  • AttrW-only (write, no getter) is rare → written longhand as AttrW(setter=…), no decorator.
• Datatype inferred from the getter's return annotation (unwrapping Update[T]); decorator kwargs typed Unpack[*Meta], validated against it. Getter docstring → attribute description.
• Introspected vs decorated attrs: don't shadow already-defined ones; raise on name clash.
• Built on the existing Unbound* bind machinery (fresh objects per instance, no prototype/deepcopy). Include the "FastCS for PyTango users" docs page.
• Unblocks Example 1 — hello-world: pure-soft @attr decorator device #398 (hello-world uses @attr); Example 2 — attr getter/setter via __init__ (temperature backend) #404's __init__ getter/setter can also be shown in @attr form for contrast.

Key files (§9)

src/fastcs/methods/ (Unbound* bind machinery), attributes/.

Acceptance

uv run --locked tox -e pre-commit,type-checking,tests green; #398 (hello-world) green against @attr.

ADR: 0018, 0014
Parent: #388
Blocked by: #392