add offset as meter start value

[tolot27]

This PR implements an offset value for previous customer's meter reading and respects it in combination with scale. It is explicitly not implemented in JQ for reuse with other meters.

The vzlogger template is enhanced as an example.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="plugin/getter.go" line_range="37" />
<code_context>
+		offset float64
</code_context>
<issue_to_address>
**issue (bug_risk):** Offset is applied before scaling, which conflicts with the documented offset unit (kWh) and likely leads to incorrect values.

The formula `(f - p.offset) * p.scale` treats `offset` as being in the raw unit of `f`, but the template documents it in kWh (`Zählerstartwert (kWh)`), and `scale` is likely a unit conversion (e.g., Wh → kWh). To apply the offset in kWh, this should be `f*p.scale - p.offset`. If you really intend `offset` to be in raw units, please update the template/help text to match; otherwise, update the computation to `return f*p.scale - p.offset, nil`.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Offset is applied before scaling, which conflicts with the documented offset unit (kWh) and likely leads to incorrect values.

The formula (f - p.offset) * p.scale treats offset as being in the raw unit of f, but the template documents it in kWh (Zählerstartwert (kWh)), and scale is likely a unit conversion (e.g., Wh → kWh). To apply the offset in kWh, this should be f*p.scale - p.offset. If you really intend offset to be in raw units, please update the template/help text to match; otherwise, update the computation to return f*p.scale - p.offset, nil.

[andig]

Which actual problem does this solve? Pls note that per contribution guidelines we prefer an issue before proposing actual changes.