feat(runtime-host): accept WebRTC direct upgrades

[M4n5ter]

English

Outcome

Add an experimental private-to-private WebRTC direct upgrade inside the existing Rust libp2p peer endpoint. Signaling runs over the already Noise-authenticated Circuit Relay v2 connection, and the resulting connection enters the same Swarm, PeerId, muxing, and Runtime Host application protocol.

Architecture boundary

• Standard /webrtc-signaling/0.0.1; no Maka signaling service or private wire protocol.
• The signaling connection's verified remote PeerId remains the identity authority; DTLS verifies the exchanged fingerprint before admission.
• Bounded SDP, candidate, message, and attempt lifetimes.
• Ordered DataChannel adapted to the existing libp2p stream contract.
• Explicit teardown/cancellation; the existing relayed coordination path survives a failed upgrade.
• No TURN, JS sidecar, second Swarm, second identity, durable ICE state, or UI.

The temporary dependency is pinned to immutable webrtc revision e132552fc67b84c30e63c5ce916a9a63e2484b6f, which contains the DataChannel readiness fixes documented in #4382. It never tracks a floating branch.

This is stack 2/4 for #4382.

Verification

• Rust 1.98 cargo fmt --check, clippy with warnings denied, and native tests.
• cargo deny --locked --exclude-dev check licenses.
• Generated Cargo dependency inventory and third-party notices are current.
• Focused loopback proves the existing application stream over an authenticated WebRTC connection.
• Phase 0 reproduction and cross-implementation evidence: [Tracking] WebRTC ICE as a second Runtime Host direct path #4382.

中文

结果

在现有 Rust libp2p peer endpoint 内加入实验性的 private-to-private WebRTC 直连升级。Signaling 复用已经通过 Noise 认证的 Circuit Relay v2 connection；建立后的 connection 进入同一个 Swarm、PeerId、mux 与 Runtime Host 应用协议。

架构边界

• 使用标准 /webrtc-signaling/0.0.1，不增加 Maka signaling service 或私有 wire。
• 已认证 signaling connection 的 remote PeerId 仍是身份 authority；DTLS 在准入前验证交换的 fingerprint。
• SDP、candidate、message 与 attempt 生命周期全部有界。
• Ordered DataChannel 适配既有 libp2p stream contract。
• 明确 teardown/cancel；升级失败不破坏已有 relay coordination path。
• 不加入 TURN、JS sidecar、第二 Swarm、第二身份、持久 ICE 状态或 UI。

临时依赖精确固定到 webrtc revision e132552fc67b84c30e63c5ce916a9a63e2484b6f；它包含 #4382 记录的 DataChannel readiness 修复，且不会跟随浮动分支。

这是 #4382 的第 2/4 层。

验证

• Rust 1.98 cargo fmt --check、clippy 零警告与 native tests。
• cargo deny --locked --exclude-dev check licenses。
• Cargo 依赖清单与第三方 notices 已更新并通过检查。
• 聚焦 loopback 证明已认证 WebRTC connection 能运行既有 application stream。
• Phase 0 复现与跨实现证据见 [Tracking] WebRTC ICE as a second Runtime Host direct path #4382。

AI use

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

Tool(s) and scope: OpenAI Codex implemented, qualified, and documented the native WebRTC transport under human direction. The human contributor owns review and merge decisions.

Checklist

• Tests cover the decision-relevant behavior
• Formatting, lint, licenses, notices, and affected tests pass locally

[jackwener]

Approved at exact head e1f2022932cdd9d18698d26d9acdd4be1efa13a3 at the explicit direction of M4n5ter. No technical review was performed as part of this action; M4n5ter requested the approval and accepts responsibility for subsequent handling.

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Review notice: This approval was submitted by an automated review agent operated by WAWQAQ and is published at the direction of M4n5ter, who requested this action and is the human accountable for it.