feat: expose runtime observability diagnostics

[harryisfish]

Summary

• expose core health probe time, consecutive failures, recovery attempts, latest failure, and real outbound egress delay in status JSON
• add myproxyctl monitor JSONL/human polling and select Mihomo/application log streams
• record proxy group member changes and persist Mihomo logs across restarts with rotation
• flag contradictory System Extension/DNS observations such as capture enabled while the provider is disabled

Verification

• cargo check --quiet
• isolated preflight pure Rust checks: 92 passed, 2 intentionally skipped
• git diff --check
• MYPROXY_DATA_DIR=/tmp/myproxy-observe-final cargo run --quiet --bin myproxyctl -- --json monitor --once

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