feat(server): split start wait stop lifecycle

[SunSi12138]

Closes #646

Summary

• Replace public Server RunAsync with StartAsync, WaitForShutdownAsync, and StopAsync as the single canonical lifecycle model.
• Expose SharpLinkServerLifecycleState independently from local HealthStatus.
• Make SharpLinkServer own and observe its accept loop and fatal runtime tasks after startup.
• Preserve connection/session cleanup as a local failure domain instead of faulting the whole Server.
• Simplify Generic Host integration to direct Server Start/Stop bridging; remove the hosted _runTask, _runCts, and ObserveRunTaskAsync lifetime model.
• Migrate demos, samples, test harnesses, benchmarks, smoke tests, and documentation away from Server RunAsync.
• Keep development versions unchanged: no package/project/assembly/file/diagnostics schema version bump.

Lifecycle semantics

• StartAsync returns only after the Server-owned accept infrastructure is established and lifecycle is published as Running; an immediate accept/startup failure is propagated to the startup caller.
• A startup caller token stops owning anything after startup succeeds. A concurrent Start waiter can cancel only its own wait.
• WaitForShutdownAsync is a pure terminal observer. Its token cancels only that caller's wait and never initiates shutdown.
• StopAsync creates or joins one shared Server-owned drain/cleanup operation. Caller cancellation cancels only the caller's wait, not the shared shutdown.
• StopAsync from Created reaches a real terminal Stopped state.
• Unrecoverable accept/heartbeat-supervisor/decode-executor termination converges on the shared terminal cleanup path and Faulted; Stop/Wait expose the same terminal failure.

Failure domains

• Accept loop / heartbeat supervisor / persistent decode executor unexpected termination: Server-terminal.
• Accepted connection lifecycle and heartbeat-timeout connection cleanup failures: observed/logged locally; the Server remains running.
• Fatal-runtime owners explicitly observe the shared terminal cleanup task. The shared _stopTask remains faulted/joinable so later StopAsync callers still receive the same terminal failure, while WaitForShutdownAsync remains an independent terminal observer.

Generic Host

• IHostedService.StartAsync awaits server.StartAsync and publishes readiness only after startup succeeds.
• IHostedService.StopAsync joins server.StopAsync without passing the Host caller token into the Server-owned stop operation.
• The Host keeps only an observer for unexpected terminal Server failure so the application lifetime can stop; it does not coordinate a second Server runtime.

Validation

Development validation was run from exact dev base 2f16532e4c18acff922d208eb262aec4999b33ac:

• exact-base / patch / public API / version+schema guards: passed
• dotnet format whitespace + git diff --check: passed
• maintainability tests and debt gate: passed
• Debug build: passed
• Release build: passed
• generated-assembly dependency verification: passed
• Unit tests: passed
• Generator tests: passed
• LoadTest tests: passed
• serialized Integration tests: passed (451/451)

Dev validation run: 34543816894.

Review follow-up validation:

• added FatalRuntimeFailureWaitOnlyShouldObserveOwnedTerminalCleanupTask, covering StartAsync -> WaitForShutdownAsync without a follow-up StopAsync and asserting the Server-owned terminal cleanup observer completes successfully;
• review-fix UnitTests, scope guard, git diff --check, and version/schema guard: passed;
• review thread resolved after the exact-head checks completed.

Final exact-head PR CI on a7f685052a8c18b01b7fc8d9d3e89f2f40280ca5 is green:

• PR Quick: passed (including Debug/Release build, Unit, Generator, LoadTest, Integration, NativeAOT, package, demo/load/chaos smoke)
• PR Fast: passed
• Extension fault containment: passed
• Pending and codec validation: passed
• Codec Padding Security Evidence: passed
• CodeQL: passed

Final branch is one commit ahead of exact dev base and contains no temporary review workflow files. No version/schema bump was introduced.

[SunSi12138]

整体方向与 #646 一致，#648 两轮 failure-domain 修复也保留住了；exact-head CI 和 base ancestry 当前都没有问题。这里还有一个 Server-owned terminal cleanup task 的 observation 缺口需要先收口。