feat: allow application MMS process-control runtimes

[masarray]

What changed

• add MmsCompositeAssociationRuntime so multiple per-association MMS runtimes can coexist
• add optional AssociationRuntimeFactory to IedSimulatorMmsServerOptions
• preserve the built-in RCB/reporting runtime and compose it with an application-supplied runtime
• keep the default simulator data model read-only when no application runtime claims a target
• dispose application runtimes with their MMS association
• add regression coverage for runtime write fall-through, first-owner reads, and disposal

Why

ARVREL's AVR virtual IED needs IEC 61850 process controls and writable AVR setting points while reusing the existing ARIEC61850 TPKT/COTP/ACSE/MMS server, BER dispatcher and reporting runtime. This extension lets an application claim only explicitly modeled process targets. Every unrelated write still reaches the existing read-only guard.

ARVREL consumer

ARVREL PR #89 pins commit 0e1e2de693bc3f5427f6b1f23fcc6c3e6fe1f60e from this branch and supplies a per-association AVR runtime for:

• SBO / SBOw / Oper / Cancel process controls
• ATCC1.Auto
• ATCC1.LTCBlk
• YLTC1.TapChg
• selected AVR setting writes

The application runtime queues accepted commands back to ARVREL's WPF/AVR authority thread; the MMS socket worker does not mutate the regulator engine directly.

Safety / compatibility

• no default write behavior changes
• RCB attribute handling remains owned by the existing reporting runtime
• applications opt in explicitly per association
• network transport, ACSE association and MMS BER dispatch semantics remain centralized in ARIEC61850
• if no application runtime claims a write target, the normal read-only rejection remains authoritative

Validation

• .NET CI #255 passes build and test for commit 0e1e2de693bc3f5427f6b1f23fcc6c3e6fe1f60e
• ARVREL .NET CI #533 also builds/tests successfully while consuming this exact commit
• PR remains draft until live SAS control validation is completed

[masarray]

Repository cleanup audit (2026-08-14): closing this draft as deferred, not merged. The process-control runtime remains preserved on feature/mms-process-control-runtime / head 0d0aa4e31c17f9e5a10901ad52fa75e9c4581daf, and the ARVREL consumer can continue pinning its validated commit. Current ARIEC main has advanced 15 commits beyond this PR's merge base and the PR still requires live SAS control validation. Re-port the minimal runtime extension onto current main only when that validation resumes; do not merge this stale branch directly. No branch or commit is being deleted.