perf(discovery): pipeline MMS model discovery with smart enrichment

[masarray]

Why

The ARSAS-vs-IEDScout capture audit showed that the current consumer path can amplify initial discovery into ~30.7k confirmed MMS requests (~23.7k GetVariableAccessAttributes and ~6.6k Read) while using only ~1-2 requests in flight, even though the tested association negotiated about 10 outstanding calls. The objective of this PR is to make the engine capable of an IEDScout-like pipelined discovery pattern while preserving ARIEC61850's correctness and receive-pump invariants.

What changed

Smart structural discovery

• Adds MmsClientSession.DiscoverSmartAsync and MmsSmartDiscoveryOptions.
• Enumerates the VMD/domain list once per smart discovery generation.
• Pipelines independent (domain, object-class) GetNameList chains with bounded concurrency.
• Caps the effective window by negotiated maxOutstandingCalling; unknown peers use a conservative fallback cap.
• Keeps every moreFollows/continueAfter chain strictly sequential.
• Rebuilds results in deterministic domain order after concurrent completion.
• Preserves successful domain branches when another chain returns an MMS-level failure.
• Does not perform eager per-leaf Read/GVA during structural discovery.

Association-scoped single-flight

• Adds DiscoverSmartSingleFlightAsync as the consumer-facing orchestration wrapper for repeated smart discovery on one accepted MMS association.
• Identical smart discovery profiles share one in-flight/completed wire operation while the accepted association marker, host, and port remain unchanged.
• A caller cancellation only cancels that waiter through Task.WaitAsync; it does not cancel the shared wire operation.
• Partial/faulted smart results are not retained as reusable cache, so later callers may retry rather than inheriting poisoned discovery state.
• A reconnect naturally invalidates the single-flight entry because the accepted association marker changes.

Structure-first exact type enrichment

• Adds GetVariableAccessAttributesSmartAsync.
• Probes LN$FC$DO roots first.
• Uses the returned structured TypeSpecification hierarchy when available.
• Falls back to exact leaf GVA only for roots that fail or do not describe descendants.
• Uses the same bounded association-aware request window.

Safe request pipelining

• Adds a single-writer gate to TpktClient.SendTpktAsync so concurrent confirmed requests cannot interleave TPKT frame bytes.
• This gate protects only frame writes; it does not serialize request/response lifecycles.
• The existing single MMS receive pump and invoke-ID routing remain unchanged.

Documentation

• Adds docs/SMART_MMS_DISCOVERY.md with invariants, migration example, capture baseline, and acceptance KPIs.

Compatibility

The existing DiscoverAsync and GetVariableAccessAttributesBatchAsync APIs remain unchanged. DiscoverSmartAsync remains available for callers that deliberately want an uncached generation; DiscoverSmartSingleFlightAsync is additive for consumers that need association-scoped deduplication.

Correctness / safety invariants

• one receive pump per association;
• no parallel raw/socket reads;
• invoke-ID correlation remains authoritative;
• continuation pages within one GetNameList chain are never parallelized;
• negotiated outstanding-call limit is respected when decoded;
• final published model ordering is deterministic;
• waiter cancellation does not tear down a shared discovery generation;
• partial/faulted discovery does not poison the reusable single-flight cache.

Acceptance target

This PR is designed to close the observed scheduling/orchestration gap; it does not claim to be faster than IEDScout until the same IED is re-captured after consumer integration. Acceptance should compare TTFI/TTFT, final LD/LN/FC-point/dataset counts, request count by MMS service, peak outstanding calls, partial-domain status, exact type coverage, and whether any second GetNameList sweep occurs on the same association.

Base: 3afc924c97627fe86adbe784c905e2f35dff0b1a (latest main at branch creation).

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[masarray]

R3 hardening committed at 1b7cbbe8af3dfbc2a15086cb2c3e5eff02e006bb.

Adds association-scoped DiscoverSmartSingleFlightAsync:

• identical smart profiles share one wire discovery on the accepted association;
• waiter cancellation uses Task.WaitAsync and does not cancel the shared operation;
• partial/faulted discovery is not retained as reusable cache;
• association marker + host/port changes invalidate reuse on reconnect.

.NET CI run #628 (35123807877) completed successfully: source verification, restore, build, test, packaging and artifact upload all passed.