AttrW setpoint cache, native timestamps + severity, ControllerRunner

[coretl]

Scope

• AttrW setpoint cache with a .get_setpoint() method (0016 L96) — needed for ophyd locate(), useful to all transports.
• Native timestamps (+ severity) on attribute updates: follow the Reading shape but share no code (0016 L101); severity is a fastcs enum using the same strings as EPICS (0016 L104).
• ControllerRunner: a class with start/stop (context manager only if it makes sense for the FastCS() case; ophyd-async uses start/stop; idempotency is the caller's responsibility). It owns the whole lifecycle incl. reconnect (0016 L108/L112). Extract from FastCS.serve. Document the stable interface (decision 13).

Key files (§9)

src/fastcs/attributes/, control_system.py (FastCS.serve).

Acceptance

uv run --locked tox -e pre-commit,type-checking,tests green.

ADR: 0016
Parent: #388
Blocked by: #392

[coretl]

Opened #420 for this.

ControllerRunner is extracted from FastCS.serve with setup/start/stop, native timestamps and severity ride on Update, and the stable interface is written down in docs/explanations/stable-interface.md.

Two things worth knowing:

• The setpoint cache this issue lists was already delivered by attributes: getter/setter IO rework, remove AttributeIORef/AttributeIO #412, as the .setpoint property — cached by set() before the setter runs and regardless of whether it succeeds, which is what ADR 0016's question 1 settled on. It is documented as part of the stable surface here, but there was no code to write. (The issue text says .get_setpoint(); ADR 0014's runtime-surface rename made it a property, and ADR 0016 Q1 agrees.)
• Reconnect turns out to have been dead. A scan task that raises sets _connected = False and pauses, but nothing in FastCS ever called Controller.reconnect() — so a controller that dropped out stayed paused unless its driver wired its own recovery. The runner now owns that, per the ADR, which is a behaviour change rather than a pure move; flagged for review on the PR.

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