feat(runtime-host): project peer Host reachability

[M4n5ter]

Summary

English

Project demanded Runtime Host connectivity as explicit ephemeral phases: discovery, connection, authentication, handshake, and ready waiting. Shared-Session targets remain sparse and become ready only after the authenticated Host handshake; membership or cached route knowledge alone never reports a usable Host.

Cancellation and target-generation fencing prevent a late connection attempt from publishing readiness after removal. This PR is stacked on #4274.

中文

将按需 Runtime Host 连接投影为明确的临时阶段：发现、连接、认证、握手和等待就绪。共享 Session target 保持稀疏，只有完成经过认证的 Host 握手后才会 ready；仅有 membership 或缓存路由绝不会被显示成可用 Host。

取消传播与 target generation fence 会阻止已移除连接的迟到结果发布 readiness。本 PR stack 在 #4274 之上。

Refs #3842
Refs #3843

Verification

English

• Runtime Host build passes.
• Focused native peer connection tests pass (4 tests).
• Desktop Runtime Host manager suite passes (43 tests), including cancellation and stale-generation fencing.
• Affected lint and diff checks pass.

中文

• Runtime Host 构建通过。
• 原生 peer 连接聚焦测试通过（4 项）。
• Desktop Runtime Host manager 测试通过（43 项），包括取消和过期 generation 隔离。
• 受影响文件的 lint 与 diff 检查通过。

Review focus

Connection phases are ephemeral observations, not durable Mesh or mount state. Membership must not create all-to-all application streams.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex — architecture planning, implementation, focused tests, and review.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No