[SunSi12138]

Refs #801.

Scope (unchanged after rebase)

• Delete the test-only SharpLinkClient.TransitionToForTesting(SharpLinkConnectionState) forwarding method.
• Change the actual production SharpLinkClient.TransitionTo(SharpLinkConnectionState) visibility from private to internal. Keep the complete state machine, NormalizeAvailabilityState, publication, locking, stop guard and runtime callers unchanged.
• Migrate all 10 direct calls in SharpLinkClientReadinessStateTests to client.TransitionTo(...), preserving the existing readiness race/state assertions.
• No public API, generated ABI or wire/protocol changes.

Post-#827 rebase / conflict resolution

• Current base: dev @ 965e25d542eaf4d35b234fe028f74c2b3eaf8be7, including merged #827.
• Exact rebase HEAD: bd3d7839c100bce656fcbb9500ba51c7a3a41df1, clean single commit parented directly on the current dev; the old refactor(client): retire readiness state transition test shim (#801) #833 head was replaced.
• Confirmed the deletion from refactor(client): replace dead shutdown-order algorithm with real dependency-binding tests (#801) #827 remains applied: neither SharpLinkClient.cs nor ClientAssemblyRegistry.cs contains GetShutdownDependencyOrder. In particular, the new head does not restore the former SharpLinkClient.GetShutdownDependencyOrder forwarding method.
• Inspected GitHub's actual post-rebase PR Diff: exactly 3 files changed — SharpLinkClient.Readiness.cs, SharpLinkClient.cs and SharpLinkClientReadinessStateTests.cs (+11/-14). No changes to ClientAssemblyRegistry.cs or unrelated areas.
• GitHub reports mergeable: true for this exact head.

CI / merge gate

An entirely new PR Fast, Package Smoke, allocation, C# analysis and targeted regression check set was automatically triggered for bd3d7839c100bce656fcbb9500ba51c7a3a41df1. Prior CI from old heads is not counted. Review the exact-head GitHub check results before merging; neither this rebase nor the preliminary mergeability check constitutes CI passage.

[SunSi12138]

已按 review 建议将 #833 重新变基到最新 dev，没有沿用旧分支对 SharpLinkClient.cs 的内容。

• 已合并的 refactor(client): replace dead shutdown-order algorithm with real dependency-binding tests (#801) #827 被完整保留：最终 SharpLinkClient.cs、ClientAssemblyRegistry.cs 都不再定义 GetShutdownDependencyOrder。
• 新基线：965e25d542eaf4d35b234fe028f74c2b3eaf8be7；新 HEAD：bd3d7839（直接以该 dev SHA 为 parent 的单一 clean commit）。
• 已核对 GitHub 实际 PR Diff 仅 3 文件、+11/-14：删除 TransitionToForTesting、更改实际 TransitionTo 可见性、迁移 10 处 Readiness 测试调用；不修改 ClientAssemblyRegistry.cs。
• GitHub 现报告 mergeable: true。
• 新 HEAD 的 PR Fast / Package Smoke / CodeQL 等门禁已重新触发；不能使用旧 HEAD 的 CI 成绩证明本次提交。结果以新 HEAD 的完整检查为准。

PR 未合并。