test(bus): deterministic dial-back source probes to decide #61 sufficiency

[hartsock]

What this PR does

• Adds three deterministic real-iroh dial-back "source probes" (bus.rs unit
  tests) that answer the empirical #61 question the #[ignore]d mDNS tests
  can't: what does incoming.remote_addr() report for a same-host dial, and does
  reply_dial_candidates actually reconnect to it? Each binds two real
  transport endpoints, has the responder observe the source exactly as
  accept_conn does, and dials the asker back over the candidate set.
• probe_loopback_v4/v6_source_is_dialable_and_reconnects — dial over loopback,
  observe + log the source, assert it's dialable and the dial-back reconnects.
• probe_scopeless_link_local_source_reconnects_via_loopback_fallback — the
  decisive #61 case: the exact fe80::/scope-0 (undialable) shape at the
  asker's real port; assert the candidate set drops it yet still reconnects via
  the loopback fallback over real iroh.

Test plan

• cargo test -p agent-mesh-bus --lib probe_ → 3/3 green, ~0.06s, deterministic.
• Observed on this host: loopback-v4 → 127.0.0.1:<port>, loopback-v6 →
  [::1]:<port> (both dialable).
• just check + just cov-ci green via the pre-push hook (57-test bus lib incl.
  the probes; coverage floor held — these only add coverage).

Out of scope

• Cross-host scoped link-local (does iroh preserve the recvmsg scope id so a
  scoped fe80:: source is directly dialable?) — genuinely a two-host / real-NIC
  scenario, left to the existing #[ignore]d reply_reaches_quiet_asker_via_dial_back
  real-LAN tier.
• No production change. #63's fix is untouched; this only adds decision
  evidence.

The decision this informs

For the same-host case (CI + the common quiet-peer scenario), #63's
candidate-set fix is sufficient: the reply is delivered even when iroh
surfaces the undialable scopeless-link-local source — proven deterministically,
end to end. The one remaining open question is cross-host scoped link-local,
which fundamentally needs the real-LAN tier.

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