SHM: generate stable device ids (BC)

[andig]

Fix #24768

To disable the new behaviour and keep IDs stable, set legacyId: true.

TODO

• UI option @naltatis
• backend "stable" setting
• use backend "stable" setting in UI @naltatis

[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• In the stable ID branch you’re mutating s.did in place by XORing it—copy the slice first so subsequent calls aren’t using a permanently altered base ID.
• You’re passing a byte slice to fmt.Sprintf with the sempDeviceId format (which expects an integer), so convert the derived did bytes to the appropriate numeric type before formatting.
• Accessing s.site.Loadpoints()[id] without bounds checking may panic—add validation or a safer lookup for the loadpoint.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the stable ID branch you’re mutating s.did in place by XORing it—copy the slice first so subsequent calls aren’t using a permanently altered base ID.
- You’re passing a byte slice to fmt.Sprintf with the sempDeviceId format (which expects an integer), so convert the derived did bytes to the appropriate numeric type before formatting.
- Accessing s.site.Loadpoints()[id] without bounds checking may panic—add validation or a safer lookup for the loadpoint.

## Individual Comments

### Comment 1
<location> `hems/shm/shm.go:327` </location>
<code_context>

 // deviceID combines base device id with device number
 func (s *SEMP) deviceID(id int) string {
-	// numerically add device number
-	did := append([]byte{0, 0}, s.did...)
</code_context>

<issue_to_address>
**issue (complexity):** Consider extracting the legacy and stable device ID logic into separate helper methods to simplify the main deviceID function.

Extract the two ID‐building branches into small helpers so that `deviceID` just picks one—no in‐place hashing or bit‐math:

```go
func (s *SEMP) deviceID(id int) string {
    if s.legacy {
        return s.legacyDeviceID(id)
    }
    return s.stableDeviceID(id)
}
```

Then pull out each implementation:

```go
func (s *SEMP) legacyDeviceID(id int) string {
    // old: prepend two zero bytes, add id, mask to 6 bytes
    did := append([]byte{0, 0}, s.did...)
    raw := binary.BigEndian.Uint64(did) + uint64(id)
    masked := ^uint64(0xffff<<48) & raw
    return fmt.Sprintf(sempDeviceId, s.vid, masked)
}
```

```go
func (s *SEMP) stableDeviceID(id int) string {
    // new: XOR SHA256(chargerRef) into base did
    ref := s.site.Loadpoints()[id].GetChargerRef()
    hash := sha256.Sum256([]byte(ref))

    out := make([]byte, len(s.did))
    copy(out, s.did)
    for i, b := range hash {
        out[i%len(out)] ^= b
    }

    return fmt.Sprintf(sempDeviceId, s.vid, out)
}
```

This keeps all functionality but moves the bit-fiddling and hashing out of the main path, improving readability and maintainability.
</issue_to_address>

---

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[andig]

You’re passing a byte slice to fmt.Sprintf with the sempDeviceId format (which expects an integer), so convert the derived did bytes to the appropriate numeric type before formatting.

@sourcery-ai I don't see an expected integer? See https://go.dev/play/p/TIwWPejEmQ5

[andig]

To save us the pain we might enable this option automatically internally if and only if there are no loadpoints configured yet. Not sure, where or how we would do this...

[paschdan]

Can we maybe add an optional parameter to the lp, e.g sempDeviceId that will be taken, and if not defined we calculate it?

There will be ghosts anyways, but at least we could get the mapping back that legacy users used to have?

[andig]

No. A loadpoint must know nothing about about specific devices.

[naltatis]

I'm away this week. @Maschga do you want to implement the UI bit and adjust the tests accordingly?