refactor(orbit): I.6 — split utils/orbit.ts 989 → 77 LOC across 10 modules (#678)

* refactor(orbit): extract constants, helpers, and state math

First extraction step of the utils/orbit.ts split. Pure, no-I/O code
moves into utils/orbit/ submodules; utils/orbit.ts becomes a re-export
shim so every call site outside this directory keeps importing from
'../utils/orbit' unchanged.

- utils/orbit/constants.ts: ORBIT_HEARTBEAT_ALIVE_MS,
  ORBIT_ORPHAN_TTL_MS, ORBIT_SHUFFLE_INTERVAL_MS, ORBIT_REMOVED_TTL_MS.
- utils/orbit/helpers.ts: generateSessionId, serialiseOrbitState +
  OrbitStateTooLarge, serialiseOutboxMeta, suggestionKey,
  parseOutboxPlaylistName.
- utils/orbit/stateMath.ts: patchOrbitState, maybeShuffleQueue,
  effectiveShuffleIntervalMs, computeOrbitDriftMs,
  applyOutboxSnapshotsToState, OutboxSnapshot type.

utils/orbit.ts: 989 → 806 LOC.

* refactor(orbit): extract remote I/O + share link

Pull readOrbitState / writeOrbitState / writeOrbitHeartbeat /
findSessionPlaylistId into utils/orbit/remote.ts. Pull
parseOrbitShareLink / buildOrbitShareLink + OrbitShareLink into
utils/orbit/shareLink.ts. Re-exported from the orbit.ts shim.

utils/orbit.ts: 806 → 721 LOC.

* refactor(orbit): extract host lifecycle

Pull startOrbitSession, endOrbitSession, triggerOrbitShuffleNow,
updateOrbitSettings, hostEnqueueToOrbit (and the StartOrbitArgs
interface) into utils/orbit/host.ts. Re-exported from the orbit.ts shim.

utils/orbit.ts: 721 → 546 LOC.

* refactor(orbit): extract host moderation

Pull kickOrbitParticipant, removeOrbitParticipant, and
setOrbitSuggestionBlocked into utils/orbit/moderation.ts. Re-exported
from the orbit.ts shim.

utils/orbit.ts: 546 → 422 LOC.

* refactor(orbit): extract guest lifecycle + suggest pipeline

Pull joinOrbitSession / leaveOrbitSession (+ OrbitJoinError), the suggest
gate (evaluateOrbitSuggestGate / OrbitSuggestGateReason /
OrbitSuggestBlockedError / suggestOrbitTrack), and the host-side
approve / decline reactions into utils/orbit/guest.ts. Re-exported from
the orbit.ts shim.

utils/orbit.ts: 422 → 244 LOC.

* refactor(orbit): extract sweep + cleanup, collapse shim

Pull sweepGuestOutboxes (+ private listGuestOutboxes / readOutbox) into
utils/orbit/sweep.ts and cleanupOrphanedOrbitPlaylists into
utils/orbit/cleanup.ts.

utils/orbit.ts collapses to a pure re-export shim — every external import
path stays the same, the file just lists which symbol lives in which
submodule.

utils/orbit.ts: 244 → 77 LOC.
Full split: 989 LOC monolith → 10 single-concern modules, none over 210 LOC.

src/utils/orbit/constants.ts

@@ -0,0 +1,35 @@
+/**
+ * Orbit cadence + TTL constants.
+ *
+ * Centralised so host/guest/state-math code reads from the same source of
+ * truth. Defaults are deliberately chosen against observed Navidrome poll
+ * cadences and real-world flake budgets.
+ */
+
+/** How long we consider a heartbeat still fresh. Longer than the guest tick so a single missed beat is tolerated. */
+export const ORBIT_HEARTBEAT_ALIVE_MS = 30_000;
+
+/**
+ * Grace window for the app-start orphan sweep. A session on the user's
+ * other device or a browser that briefly restarted must NOT be deleted
+ * by this sweep. 5 min matches the guest-side host-timeout threshold:
+ * if a session is silent for that long, it's fair to treat it as dead;
+ * anything shorter is a real restart and must survive.
+ */
+export const ORBIT_ORPHAN_TTL_MS = 5 * 60_000;
+
+/**
+ * Legacy / fallback shuffle cadence. New sessions store their own interval
+ * in `OrbitState.settings.shuffleIntervalMin`; `effectiveShuffleIntervalMs`
+ * resolves that against this constant for sessions created before the
+ * field existed.
+ */
+export const ORBIT_SHUFFLE_INTERVAL_MS = 15 * 60_000;
+
+/**
+ * How long a soft-`removed` marker stays in the state blob. Long enough for
+ * the affected guest's 2.5 s read tick to surface the modal even after a
+ * one-tick miss; short enough that the marker doesn't bloat state if the
+ * guest never reconnects.
+ */
+export const ORBIT_REMOVED_TTL_MS = 60_000;