Add full web-port plan (WebSockets + WebRTC, no ENet)

[ConstCorrectness]

Motivation

• Provide a concrete migration plan to port AssaultCube to web browsers without ENet by defining networking, voice, and rendering approaches for a browser-native client.
• Create a solo-friendly roadmap and task list to guide implementation, scope decisions, and early milestones.

Description

• Add docs/web_port_plan.md describing the target architecture with a WASM core client, JavaScript host integrations, native authoritative server, and a WebSocket session layer replacing ENet in browser paths.
• Define a binary WebSocket protocol envelope, ENet-like application-layer reliability classes, and a server-authoritative tick/prediction/reconciliation model for gameplay.
• Specify a WebRTC-based voice plan (SFU-first), reuse WebSocket signaling for SDP/ICE, outline the audio capture/processing pipeline, and enumerate moderation/UX controls.
• Outline rendering migration to WebGL2, material/shader considerations, security recommendations, milestone roadmap, immediate task list, and explicit non-goals for the initial launch.

Testing

• Validated the new file contents with nl -ba docs/web_port_plan.md | sed -n '1,220p', which displayed the expected planning document.
• Confirmed repository state and PR metadata were produced by the PR tooling and that the added planning document is present and accessible.

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Codex Task