feat(p2p-06): add p2p service lifecycle shell (#671)

[jlon]

Summary

• Introduce P2pService lifecycle ownership in the client filesystem stack.
• Keep reads unchanged; this PR only establishes start/stop and stats shell behavior.

Design

• Model service state explicitly as Disabled, Stopped, and Running.
• Keep ownership in FsContext and bind startup/shutdown to filesystem lifecycle boundaries.

Implementation

• Add P2pService, P2pState, and stats snapshot exports.
• Construct the service from config and hook filesystem start/stop plus basic lifecycle tests.

Verification

• cargo test -p curvine-client disabled_service_does_not_start
• cargo test -p curvine-client enabled_service_transitions_between_stopped_and_running
• cargo test -p curvine-client fs_context_creates_p2p_service_when_enabled

[jlon]

Part 6/17 of the split P2P series that supersedes #671.

This PR targets , so the visible diff is cumulative. Review the slice described in this PR body instead of treating the whole diff as new.

Intended review delta is p2p-06 on top of #758.

[jlon]

Review note: this is part 6/17 of the split P2P series that supersedes #671.

This PR targets main, so the visible diff is cumulative. Please review the slice described in this PR body.

Review after #758.