feat(guest): turn-based guest participation (fix iOS guest never replying)

[boggspa]

Problem

Guest chats (host + one attached guest) worked on desktop but the guest never replied on iOS — you could add/configure a guest, but only starting a side-chat with it directly produced a response.

Root cause (GAP B from the 2026-06-16 guest-parity review): the guest trigger + reply-mirror lived only in the renderer (App.tsx). iOS-origin turns run through the Mac bridge's composerPromptFn, which never touches the renderer — so it only ever dispatched the host. The bridge didn't even tell the host a guest was attached.

Separately, desktop was a parallel fan-out (host and guest fired together), so the guest never saw the host's reply to the same turn.

Fix — turn-based guest participation on both platforms

Per the chosen behaviour: host responds first, then the guest (so the guest sees the host's reply); an @-tag aims the prompt at only that agent (ensemble-style).

• New shared module src/main/GuestParticipantRun.ts (node-safe, unit-tested): steering preamble, parent-transcript peer-context builder, full guest-prompt composer, and the guest-return-${runId} reply-mirror message builder (deduped by guestRunId). Provider-label injected so it's tree-agnostic.
• Bridge (the iOS fix) — composerPromptFn now passes guestParticipant to the host prompt (host-awareness), routes @-tags (@guest → guest only, @parent/@host → host only, no tag → host then guest), and after the host run finalizes dispatches the guest as a normal leaf run on its child chat (so it answers with the host's reply in context), then mirrors the guest's reply into the parent transcript on the guest run's finalize. Wired via a module-ref runner assigned in the bridge closure so the module-scope finalizeBridgeRunTranscript can reach the closure dispatch helpers.
• Renderer (parity) — the fan-out now defers the guest: it's dispatched from the run-completion handler once the host run on that chat finishes (with a fresh chatRecord), instead of firing in parallel. @guest stays immediate; @parent/@host stays host-only.

iOS needs no changes — it already renders guestParticipantReply provider-tinted/attributed (GAP A, cb74513a) and excludes the guest from the side-chats tab (GAP C, 56899180). Once the Mac mirrors + broadcasts the guest reply, the phone shows it.

Verification

• npm run typecheck:node + npm run build (main/preload/renderer) — green; confirms the cross-tree extractGuestParticipantAddressTarget import bundles into main.
• Full vitest suite: 405 files / 4838 tests passing.
• New unit tests: GuestParticipantRun.test.ts (13). @-routing already covered by ComposerMentionTrigger.test.ts; host guest-context by PromptComposition.test.ts.

Remaining verification (not in this PR)

• Manual device test: on a paired iPhone, send to a guest chat → host replies, then the guest reply appears in the parent transcript; @guest / @parent route correctly.
• Optional fake-provider e2e for the bridge host→finalize→guest→mirror chain (the bridge wiring is integration-level; the pure logic is unit-tested). Happy to add it if wanted before merge.

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