Read active alarms over native S7CommPlus

[ale-rinaldi]

Read active alarms over native S7CommPlus

The C# reference driver reads the PLC alarm system natively; python-snap7 has no alarm support. Add reading of the active-alarm list (and the alarm catalog/texts).

Goal

An API like client.read_alarms(language_id=...) returning active alarms: {alarm_id, state (Coming/Going), text, additional texts, timestamp, associated values}.

Approach (mirror the C# Alarming/ package)

1. Browse the alarm catalog: EXPLORE the alarm subsystem (Ids.NativeObjects_theAlarmSubsystem_Rid = 8, already NATIVE_THE_ALARM_SUBSYSTEM_RID in protocol.py) to enumerate alarm definitions and their per-language texts.
2. Parse alarm state: decode the CGS/DAI state objects → active alarms. Subtype 2673 = DAI.Coming, 2677 = DAI.Going.
3. Alarm texts: per LanguageId (AlarmText + AdditionalText1..9, Infotext); resolve associated values into the text.

C# reference (thomas-v2/S7CommPlusDriver, LGPL-3.0) — src/S7CommPlusDriver/Alarming/

• BrowseAlarms.cs — enumerate the catalog
• AlarmsDai.cs, AlarmsAsCgs.cs — state objects (FromNotificationObject)
• AlarmsAlarmTexts.cs, AlarmsAssociatedValues.cs, AlarmsHmiInfo.cs
• AlarmsHandler.cs — orchestration

Already in place

• Corrected V2 wire format, TLS, EXPLORE + fragment reassembly (fix(s7commplus): correct V2 symbolic reads on a real S7-1500 (wire format + async/server TLS-in-COTP) gijzelaerr/python-snap7#745/fix(s7commplus): restrict TLS groups in-code so the handshake works on OpenSSL >= 3.5 gijzelaerr/python-snap7#746/feat(s7commplus): enumerate datablocks via EXPLORE (request fix + fragment reassembly) gijzelaerr/python-snap7#749) — the transport this needs.
• IDs: NATIVE_THE_ALARM_SUBSYSTEM_RID, ALARM_SUBSCRIPTION_* in protocol.py.

Validation

Compare the active set against the alarms an HMI shows as active for the same PLC; the native list should match.

Note: live-pushed alarm notifications belong to the subscriptions issue; this issue is the polled/active-list read.