refactor(decouple): orbit seam — invert orbit off the audio core

Add core seam store/orbitRuntime.ts: a registry (registerOrbitRuntime) exposing a
neutral session snapshot {role,phase,state} + an async bulkGuard, plus pure
derivations mirrored from the feature (isInOrbitSession, isOrbitPlaybackSyncActive,
estimateLivePosition). Default (unregistered) = neutral snapshot + bulkGuard allow
— bit-identical to today's no-session behavior, and a session can only start via
the topbar which loads the @/features/orbit barrel (→ registers) first.

features/orbit/utils/orbitBulkGuard registers the runtime at module init
(store-backed getSnapshot + the existing confirm-modal orbitBulkGuard as the gate).
The orbit feature keeps its own copies of the pure helpers for UI (incl. the
arg-form isOrbitPlaybackSyncActive(role,phase) used by two settings/player-bar
components), so nothing UI-facing changes.

Repointed the 9 audio-core sites (playbackRateStore, previewStore,
playbackReportSession, nextAction, resumeAction, playTrackAction,
queueMutationActions, playAlbum) from @/features/orbit to @/store/orbitRuntime;
state reads (useOrbitStore.getState().role/.state) become orbitSnapshot().
Migrated 6 audio-core test mocks to @/store/orbitRuntime via importOriginal-spread
(keeps registerOrbitRuntime callable). enqueueShareSearchPayload stays on the orbit
barrel (share util, not audio core) — its test mock unchanged.

Decouple Step 3 — last seam. The audio ENGINE is now free of @/features/* runtime
imports (only type-only edges + the non-engine composerBrowseSessionStore browse
store remain). Unblocks the playback-core move + utils/library→lib.

tsc 0, lint 0, full suite 319/2353 green.
NEEDS Frank's live-session QA before relying on it (host+guest bulk-replace modal,
guest catch-up, rate/preview/scrobble suppression).

src/store/nextAction.ts

@@ -9,7 +9,7 @@ import {
   isInfiniteQueueFetching,
   setInfiniteQueueFetching,
 } from './infiniteQueueState';
-import { isInOrbitSession } from '@/features/orbit';
+import { isInOrbitSession } from '@/store/orbitRuntime';
 import type { PlayerState, QueueItemRef, Track } from './playerStoreTypes';
 import { toQueueItemRefs } from '../utils/library/queueItemRef';
 import { resolveQueueTrack } from '../utils/library/queueTrackView';