doctor: add --skip-native-check flag

[dolonet]

Summary

Adds a --skip-native-check flag to mtg doctor that skips the Validate native network connectivity section.

When proxy chaining is configured (network.proxies), the native dialer is intentionally not the path used to reach Telegram DCs, so this section currently times out for every DC. #485 makes those dials concurrent (worst case ~10s instead of ~60s), but for users who know native egress is broken by design, even a single 10s window is wasted time on every mtg doctor invocation.

The check is still useful by default (it confirms the host can reach Telegram directly when no proxy is configured), so the behavior is opt-in.

Sample output with the flag set:

Validate native network connectivity
  ⏭ Skipped (--skip-native-check)
Validate network connectivity with proxy ...
  ✅ DC 1
  ...

Scope is intentionally narrow — only checkNetwork(base) is gated. checkFrontingDomain also uses the native dialer, but it is conceptually distinct (it tests whether the proxy host itself can reach the fronting domain, which is still meaningful under chain-mode), so it is left untouched.

This re-sends the same diff as the previously closed #484 — closed it prematurely thinking #485 alone would be enough; the issue reporter clarified in #482 (comment) that both the speedup and the skip option are wanted. Independent of #485 and can land in either order.

Closes #482.

Test plan

• Diff is byte-for-byte identical to closed doctor: add --skip-native-check flag #484 which was mergeable_state: clean
• Manual verification: run mtg doctor --skip-native-check config.toml against a chained-proxy config, confirm the native section is skipped and the rest of the report runs normally

[9seconds]

Yes, makes sense. This part of the project has to be revisited though: this is not enough to check for simple socket opening, we also need to ping those servers with RPC pings. Current verification is a very shallow unfortunately.

Thank you!