cite IEC 61131-3 clause numbers in diagnostics

[michaeldistel]

Summary

Include IEC 61131-3 clause/section references in diagnostic messages and a setting to toggle them.

Scope

• Each diagnostic produced by the server includes a [IEC §X.Y.Z] suffix when the setting is enabled
• New setting: controlforge-st.showIECReferences (boolean, default false)
• Applies to existing and future diagnostics where a spec clause is known

Implementation Notes

• Add clause metadata to each diagnostic definition in src/server/
• Read setting from workspace config in server.ts
• Setting change triggers re-validation

Acceptance

• With setting on, diagnostic messages show IEC clause reference
• With setting off, messages are unchanged
• Clause numbers sourced from docs/IEC61131_SPECIFICATION.md
• Tests cover setting on/off for at least 3 diagnostic types