feat(protocol): add schema-pinned Codex app-server framing and initialization

[hartsock]

Part of #1805. Depends on #1806.

Goal

Add a protocol-only package for the pinned Core Remote Coding v1 contract, with exact framing, IDs, errors, and initialization behavior and no dependency on the Newt runtime.

Scope

• Add minimal typed request, response, notification, server-request, and error envelopes generated from or continuously checked against the pinned schema.
• Implement newline-delimited app-server framing, including omission of the standard JSON-RPC jsonrpc member.
• Preserve every request-ID form admitted by the pinned contract.
• Implement the connection state machine: reject traffic before initialize, accept one initialization, process initialized, and match the reference behavior for reinitialization.
• Honor the pinned client capability fields, including exact notification opt-outs and experimental opt-in behavior when present.
• Return reference-compatible parse, invalid-request, method-not-found, invalid-params, and internal errors.
• Bound frame sizes and reject malformed or unsupported variants without panics.
• Keep the supported Rust types deliberately smaller than the upstream universe; unsupported surface remains explicit in the compatibility matrix.

Acceptance criteria

• Every golden fixture round-trips where the reference round-trips.
• Emitted frames validate against the pinned generated schema and never contain a jsonrpc member.
• Pre-initialize, initialize, initialized, duplicate-initialize, unknown-method, bad-ID, and invalid-params tests match the normalized reference traces.
• Experimental descriptors are rejected unless the client opted in and the profile implements them.
• Property/fuzz tests cannot panic or allocate without the configured bound on arbitrary input.
• The package has no dependency on newt-core, inference, tools, TUI, ACP, MCP, Agent Mesh, or Agent Bridle.

Out of scope

• Transport processes or sockets.
• Thread/turn behavior.
• Newt extension methods.
• Inventing a protocol version field that the upstream handshake does not define.