HEMS: add FNN 3-relay (experimental)

[andig]

Depends on #25887

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The maxPower field is effectively unused (only referenced in commented code in curtail); either wire it up via SetMaxPower or remove the parameter/field to avoid confusion.
• Run uses time.Tick without any way to stop the goroutine, which can leak a ticker and run forever; consider using time.NewTicker with a stop mechanism (e.g., context cancellation) to allow clean shutdown.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `maxPower` field is effectively unused (only referenced in commented code in `curtail`); either wire it up via `SetMaxPower` or remove the parameter/field to avoid confusion.
- `Run` uses `time.Tick` without any way to stop the goroutine, which can leak a ticker and run forever; consider using `time.NewTicker` with a stop mechanism (e.g., context cancellation) to allow clean shutdown.

## Individual Comments

### Comment 1
<location> `hems/fnn/fnn-3.go:46-53` </location>
<code_context>
+		return nil, errors.New("hems requires load management- please configure root circuit")
+	}
+
+	// register LPP circuit if not already registered
+	lpp, err := shared.GetOrCreateCircuit("lpp", "fnn-3")
+	if err != nil {
+		return nil, err
+	}
+
+	// wrap old root with new pc parent
+	if err := root.Wrap(lpp); err != nil {
+		return nil, err
+	}
</code_context>

<issue_to_address>
**issue (bug_risk):** Avoid performing irreversible side effects (wrapping root circuit) before all dependent getters are successfully created

If `cc.S1.BoolGetter`, `cc.S2.BoolGetter`, or `cc.W3.BoolGetter` fails, this function returns an error after `root.Wrap(lpp)` and `site.SetCircuit(lpp)` have already changed global state, leaving a partially configured system despite `NewFromConfig` failing. Please reorder so all getters are constructed and configuration validated before wrapping the root and calling `site.SetCircuit`, or add compensating logic to roll back those side effects on error.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Avoid performing irreversible side effects (wrapping root circuit) before all dependent getters are successfully created

If cc.S1.BoolGetter, cc.S2.BoolGetter, or cc.W3.BoolGetter fails, this function returns an error after root.Wrap(lpp) and site.SetCircuit(lpp) have already changed global state, leaving a partially configured system despite NewFromConfig failing. Please reorder so all getters are constructed and configuration validated before wrapping the root and calling site.SetCircuit, or add compensating logic to roll back those side effects on error.