feat(sessions): add durable observation archive and replay API

tests/test_active_session_lease.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from k1link.sessions import ActiveSessionLease, recover_stale_active_session_marker
+from k1link.sessions.legacy import discover_legacy_viewer_sessions
+
+
+def test_active_session_lease_hides_live_evidence_until_release(tmp_path: Path) -> None:
+    sessions = tmp_path / "sessions"
+    session = sessions / "20260717T011050Z_viewer_live"
+
+    lease = ActiveSessionLease.acquire(sessions, session)
+    try:
+        session.mkdir()
+        capture = session / "captures" / "mqtt_live"
+        capture.mkdir(parents=True)
+        (capture / "mqtt.raw.k1mqtt").write_bytes(b"K1MQTT\x00unfinished")
+        candidates = discover_legacy_viewer_sessions(sessions)
+        assert candidates == ()
+        assert recover_stale_active_session_marker(sessions) is False
+    finally:
+        lease.release()
+
+    assert not (sessions / ".current_session").exists()
+    candidates = discover_legacy_viewer_sessions(sessions)
+    assert len(candidates) == 1
+    assert candidates[0].session_id == session.name
+    assert candidates[0].replayable is False
+
+
+def test_startup_recovery_removes_only_an_unlocked_stale_marker(tmp_path: Path) -> None:
+    sessions = tmp_path / "sessions"
+    sessions.mkdir()
+    marker = sessions / ".current_session"
+    marker.write_text("20260717T011050Z_viewer_live\n", encoding="utf-8")
+
+    assert recover_stale_active_session_marker(sessions) is True
+    assert not marker.exists()
+    assert recover_stale_active_session_marker(sessions) is False