Set up review-protocol client skeleton for `roxyd`

[sae-bom]

Background

Based on the Manager <-> roxyd design discussed in #519
This issue sets up the review-protocol client-side connection skeleton for roxyd.

This is scaffolding-only.
No RequestCode handling or privileged operations are implemented in this issue.

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Scope

In scope

• Connection lifecycle structure (e.g. connect -> run)
• A minimal mock Manager for tests (handshake-only)

Out of scope

• Implementing any RequestCode-specific logic
• Implementing or porting existing handlers
• Performing privileged operations
• Introducing retries, backpressure, or rate limiting
• Feature-flag–based switching logic
• Observability, metrics, or health endpoints

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Tasks

1. Connection struct

• Introduce a connection-managing struct.
• The struct owns the review-protocol connection and related state.

2. Connect phase

• Implement connect(...) -> Result<Self> (or equivalent) that:
  • establishes QUIC/mTLS connectivity
  • performs the initial handshake
  • returns a connected instance
• Use review_protocol::client::ConnectionBuilder::new() as the single entrypoint for establishing the connection (no direct QUIC wiring in this issue).

3. Run loop

• Implement run(self) -> Result<()> (or equivalent) that:
  • receives messages from Manager
  • delegates message handling to a separate function

4. Dispatch boundary

• Add a function such as handle_request(...) -> Result<()> that:
  • logs the received message / RequestCode
  • includes an explicit match statement that enumerates exactly these RequestCodes:
    • Reboot
    • Shutdown
    • ResourceUsage
    • ProcessList
  • for every RequestCode arm, fail explicitly with unimplemented!() (scaffolding-only)
  • for all other RequestCodes, use a fallback arm (e.g. _ => ...) that also fails explicitly with unimplemented!()

5. Tests (mock Manager)

• Add a test that spins up a minimal mock Manager (review-protocol server) and verifies:
  • QUIC/mTLS connection establishment using review_protocol::client::ConnectionBuilder::new(), and
  • completion of the initial handshake.
• The mock Manager should implement only the minimum required for handshake (no RequestCode business logic).
• Keep tests deterministic and self-contained (no external network dependencies).

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Notes

• The review-protocol repository is here: https://github.com/petabi/review-protocol.

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Acceptance Criteria

• A review-protocol client connection-managing skeleton exists for roxyd and compiles successfully.
• connect() establishes connectivity and handshake.
• Incoming RequestCodes are received and dispatched to handler (handle_request()).
• All handler paths fail explicitly with unimplemented!().
• No real handler logic or privileged operations are introduced.
• A unit/integration test exists that uses a minimal mock Manager to validate QUIC/mTLS + handshake.
• CI passes