Split server start/wait/stop lifecycle

[SunSi12138]

Motivation

Fixes #646 by making Server lifecycle explicit and single-owner: startup, long-running serving, terminal waiting, and shutdown are no longer combined in public RunAsync.

Changes

• Replace public RunAsync with StartAsync, WaitForShutdownAsync, and StopAsync as the only canonical Server lifecycle primitives.
• Expose SharpLinkServerLifecycleState independently from HealthStatus.
• Move accept-loop and Server-runtime background-task ownership/terminal failure handling into SharpLinkServer.
• Keep per-connection and deferred-cleanup task failures inside their local failure domain: they remain observed/logged and joined during shutdown, but do not independently fault the whole Server.
• Keep heartbeat-timeout connection cleanup inside the connection failure domain while retaining heartbeat-supervisor failure as terminal.
• Make WaitForShutdownAsync a pure terminal observer; caller cancellation only cancels that wait.
• Keep StopAsync as one shared Server-owned shutdown operation; caller cancellation does not cancel or force the shared drain.
• Simplify Generic Host integration: no _runTask / _runCts; Host Start/Stop bridge directly to Server Start/Stop, with only a terminal observer for unexpected Server termination. Hosted Stop callers also only cancel their own wait.
• Migrate demos, integration/AOT/load/chaos/package-smoke harnesses to the new primitives without adding a public compatibility shim.
• Document lifecycle/readiness separation and add the Unreleased changelog entry.

Lifecycle semantics

• StartAsync publishes Running only after the Server-owned accept operation has established its startup boundary; an immediate listener/accept failure is surfaced to the startup caller.
• The built-in socket listener still performs bind/listen eagerly when the transport is constructed, so bind errors remain fail-fast before StartAsync; they are not deferred to a background task.
• After startup, the accept supervisor and fatal Server-runtime tasks are owned and observed by SharpLinkServer; unrecoverable runtime failures converge on one terminal cleanup path and Faulted state.
• Per-connection lifecycle/cleanup faults are observed without widening their failure domain to the whole Server.
• Heartbeat timeout disconnects isolate/log connection cleanup failures at the per-connection boundary; other unhandled heartbeat-supervisor failures remain terminal.
• WaitForShutdownAsync never initiates shutdown and completes/faults only when terminal cleanup is published.
• StopAsync owns Draining -> Stopped/Faulted; all callers join the same operation and terminal failures are consistent with WaitForShutdownAsync.
• StopAsync from Created produces a real terminal Stopped state; startup cancellation does not become a long-lived Server lifetime token after Running is published.

Validation

Final head: b3b372a45fb1baae1e1fa4ffe7e1145d644f0702 (single commit on current main).

• PR Quick #2809: passed — Release build, Unit, Generator, LoadTest, Integration, AOT smoke, pack/package verification, demo/load/chaos smoke.
• Release Gate [P1][Client] Deadline scan can target a reused PendingCall generation #560: passed — Linux, Windows, and macOS build/test matrices; Linux/Windows/macOS AOT; Linux/Windows/macOS chaos; pack/package smoke.
• CodeQL #3846: passed.
• Review follow-up: split tracked-task observation into non-terminal TrackFrameworkTask and terminal TrackServerRuntimeTask; per-connection/deferred cleanup keeps the local policy.
• Heartbeat review follow-up: DisconnectHeartbeatTimedOutConnectionAsync isolates/logs heartbeat-timeout connection cleanup faults without downgrading the heartbeat supervisor itself.
• Regression coverage: ConnectionCleanupFailureShouldNotTerminateServerRuntime and HeartbeatCleanupFailureShouldNotTerminateServerRuntime; both verify local cleanup failure does not terminate the Server, with the latter also proving accept continues after heartbeat cleanup failure.
• Self-review: no Server public RunAsync compatibility entry remains; Generic Host has no run-loop CTS/task and no caller-cancellation ForceStop path; lifecycle coordination is isolated in SharpLinkServer.PublicLifecycle.cs; no temporary issue-646 workflow/script files remain in the PR tree.
• API migration was cross-checked with [Runtime Architecture][Phase 18] 完成 public API/package 审计、发布门禁与迁移文档 #86: this is an intentional breaking lifecycle migration, with no long-lived legacy forwarding adapter.

[SunSi12138]

整体 Start/Wait/Stop 分域、Host bridge 和 shared-stop cancellation 方向合理，exact-head Quick / Release Gate / CodeQL 也已通过。当前有一个 failure-domain blocker：本 PR 把共享 TrackFrameworkTask 中的所有异常都升级为 Server terminal failure，但该 tracker 同时承载 per-connection lifecycle task，而连接 cleanup 明确允许抛异常；这会让单个坏连接/transport cleanup 把整个 Server 置为 Faulted。见 inline。