fix(calls): bind the Online HFP modem (skip stale offline modems)

[quad341]

Follow-up to #132. CallController._discover_modem bound the first MAC-matching iPhone HFP modem without checking Online; with the iPhone HFP-connected on two adapters it could bind an offline modem (no VoiceCallManager), so every Dial/Answer failed with UnknownMethod. Now prefers an Online modem (sort Online-first). Proven live during a real call (Dial -> active). Regression test tracked as a follow-up bead.

[quad341]

Thanks @quad341 — this is a clean fix for a genuinely nasty failure mode. Binding the first MAC-matching modem without checking Online looks fine right up until the iPhone is HFP-connected on two adapters and you silently grab the dead one with no VoiceCallManager, so every Dial/Answer dies with UnknownMethod. Collecting the candidates, sorting Online-first (stable, so single-modem behavior is unchanged), and still falling through to retry when there are none is exactly the right shape — and the comment spelling out the UnknownMethod chain will save whoever touches this next. Nice that you proved it live with a real Dial→active.

Merging as-is — CI's green, branch is mergeable, and there's no integration work on my side.

One thing for the follow-up bead you already mentioned: _on_modem_added still binds the first HFP iPhone modem without the Online check, so the same offline-bind can sneak back in through the dynamic-add path. Worth folding into the same two-modem (one offline, one online) regression fixture when you get to it. Appreciate the tight, well-explained change.

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Reviewers: Qwen (local) — ok · Claude (claude-opus-4-8[1m]) — ok · Codex (gpt-5.5) — ok
Synthesis: claude-opus-4-8[1m]

Verified locally + tests pass. Ready to merge — clicking the button is the only step left.

[quad341]

Re-reviewed 9c64e4621..ea63ad268 (2 new commits since the last review): no new concerns — the prior review stands.