Fronius Gen24: add PV curtailment support

[tilalx]

Summary

• Adds curtail/curtailed to the fronius-gen24.yaml PV usage block, implementing api.Curtailer via SunSpec model 123 (WMaxLimPct/WMaxLim_Ena) — the same mechanism already used by sunspec-inverter-curtailable.yaml.
• Adds "curtail" to the template's capabilities list.

Test plan

Verified against real GEN24 hardware (12 kW Symo GEN24 Plus) over Modbus TCP:

• Confirmed SunSpec model 123 registers present and correctly named (WMaxLimPct, WMaxLim_Ena)
• Wrote a curtailment limit below current natural inverter output and confirmed the grid meter showed increased import, consistent with AC output actually being capped (not just the register accepting the write)
• Reverted to 100%/uncurtailed and confirmed normal behavior resumed

[sourcery-ai]

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

• In the curtail.set switch cases you have nested set keys (e.g. set: { source: const, ... set: { source: sunspec ... } }), which is likely invalid for this template DSL; consider separating these into a proper sequence of steps rather than nesting set inside another set block.
• The curtailed script returns an int derived from a float limit; if the surrounding system expects a float percentage like other PV limit fields, it may be safer to keep this as a float to avoid unintended truncation or type mismatches.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the `curtail.set` switch cases you have nested `set` keys (e.g. `set: { source: const, ... set: { source: sunspec ... } }`), which is likely invalid for this template DSL; consider separating these into a proper sequence of steps rather than nesting `set` inside another `set` block.
- The `curtailed` script returns an `int` derived from a `float` limit; if the surrounding system expects a float percentage like other PV limit fields, it may be safer to keep this as a float to avoid unintended truncation or type mismatches.

## Individual Comments

### Comment 1
<location path="templates/definition/meter/fronius-gen24.yaml" line_range="180-188" />
<code_context>
+        value: 123:WMaxLimPct
+      - source: switch
+        switch:
+          - case: 100 # Uncurtailed
+            set:
+              source: const
+              value: 0 # Limit disabled
+              set:
+                source: sunspec
+                uri: {{ joinHostPort .host .port }}
+                id: 1
+                value: 123:WMaxLim_Ena
+        default:
+          source: const
</code_context>
<issue_to_address>
**issue (bug_risk):** The nested `set` under the `case: 100` branch is structured differently from the `default` branch and is likely not parsed as intended.

In the `case: 100` branch you currently have a nested `set`:

```yaml
- case: 100 # Uncurtailed
  set:
    source: const
    value: 0 # Limit disabled
    set:
      source: sunspec
      uri: {{ joinHostPort .host .port }}
      id: 1
      value: 123:WMaxLim_Ena
```

whereas the `default` branch uses a single `set` block:

```yaml
default:
  source: const
  value: 1 # Limit enabled
  set:
    source: sunspec
    uri: {{ joinHostPort .host .port }}
    id: 1
    value: 123:WMaxLim_Ena
```

Because the `case: 100` structure differs from the expected pattern, the Sunspec write there may not execute. Please align the `case: 100` structure with the `default` branch (const source + one `set` block) so both paths behave consistently.
</issue_to_address>

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