feat: replace Git-DAG timeline with flat snapshot browser

Replace HistoryTree (DAG with branches, Graphviz rendering) with a flat
chronological SnapshotTimeline. New UI features: split-view layout,
snapshot compare/diff, cherry-pick restore of individual sequences or
fields, auto-snapshots with debounce, and pin/filter support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_snapshot_timeline.py

@@ -0,0 +1,159 @@
+import pytest
+from snapshot_timeline import SnapshotTimeline, diff_snapshots
+
+
+def test_record_creates_snapshot():
+    tl = SnapshotTimeline({})
+    sid = tl.record({"batch_data": [{"seed": 42}]}, note="first")
+    assert sid in tl.snapshots
+    assert tl.current_id == sid
+    assert tl.snapshots[sid]["note"] == "first"
+    assert tl.snapshots[sid]["auto"] is False
+    assert tl.snapshots[sid]["seq_count"] == 1
+
+
+def test_record_auto_flag():
+    tl = SnapshotTimeline({})
+    sid = tl.record({"batch_data": []}, note="auto save", auto=True)
+    assert tl.snapshots[sid]["auto"] is True
+
+
+def test_multiple_records():
+    tl = SnapshotTimeline({})
+    id1 = tl.record({"batch_data": [{"a": 1}]}, note="one")
+    id2 = tl.record({"batch_data": [{"b": 2}]}, note="two")
+    assert len(tl.snapshots) == 2
+    assert tl.current_id == id2
+
+
+def test_to_dict_roundtrip():
+    tl = SnapshotTimeline({})
+    tl.record({"batch_data": [{"x": 1}]}, note="test")
+    d = tl.to_dict()
+    tl2 = SnapshotTimeline(d)
+    assert tl2.current_id == tl.current_id
+    assert set(tl2.snapshots.keys()) == set(tl.snapshots.keys())
+
+
+def test_migrate_from_history_tree():
+    """Old HistoryTree format should be flattened into snapshots."""
+    old_data = {
+        "nodes": {
+            "aaa": {"id": "aaa", "parent": None, "timestamp": 1000, "note": "First", "data": {"batch_data": [{"seed": 1}]}},
+            "bbb": {"id": "bbb", "parent": "aaa", "timestamp": 2000, "note": "Second", "data": {"batch_data": [{"seed": 2}]}},
+        },
+        "branches": {"main": "bbb"},
+        "head_id": "bbb",
+    }
+    tl = SnapshotTimeline(old_data)
+    assert len(tl.snapshots) == 2
+    assert tl.current_id == "bbb"
+    assert tl.snapshots["aaa"]["note"] == "First"
+    assert tl.snapshots["bbb"]["note"] == "Second"
+    # Data should be preserved
+    assert tl.snapshots["aaa"]["data"]["batch_data"] == [{"seed": 1}]
+
+
+def test_migrate_from_history_tree_no_data():
+    """Slim tree nodes (no inline data) should still migrate."""
+    old_data = {
+        "nodes": {
+            "aaa": {"id": "aaa", "parent": None, "timestamp": 1000, "note": "First"},
+        },
+        "branches": {"main": "aaa"},
+        "head_id": "aaa",
+    }
+    tl = SnapshotTimeline(old_data)
+    assert len(tl.snapshots) == 1
+    assert tl.snapshots["aaa"]["seq_count"] == 0
+
+
+def test_migrate_legacy_prompt_history():
+    legacy = {
+        "prompt_history": [
+            {"note": "A", "seed": 1},
+            {"note": "B", "seed": 2},
+        ]
+    }
+    tl = SnapshotTimeline(legacy)
+    assert len(tl.snapshots) == 2
+    assert tl.current_id is not None
+
+
+def test_toggle_pin():
+    tl = SnapshotTimeline({})
+    sid = tl.record({"batch_data": []}, note="test")
+    assert tl.snapshots[sid]["pinned"] is False
+    result = tl.toggle_pin(sid)
+    assert result is True
+    assert tl.snapshots[sid]["pinned"] is True
+    result = tl.toggle_pin(sid)
+    assert result is False
+
+
+def test_delete_snapshot():
+    tl = SnapshotTimeline({})
+    id1 = tl.record({"batch_data": []}, note="one")
+    id2 = tl.record({"batch_data": []}, note="two")
+    tl.delete(id2)
+    assert id2 not in tl.snapshots
+    assert tl.current_id == id1
+
+
+def test_delete_all_snapshots():
+    tl = SnapshotTimeline({})
+    sid = tl.record({"batch_data": []}, note="only")
+    tl.delete(sid)
+    assert len(tl.snapshots) == 0
+    assert tl.current_id is None
+
+
+def test_strip_snapshots():
+    tl = SnapshotTimeline({})
+    tl.record({"batch_data": [{"a": 1}]}, note="test")
+    tl.strip_snapshots()
+    for snap in tl.snapshots.values():
+        assert "data" not in snap
+
+
+def test_get_snapshot_data():
+    tl = SnapshotTimeline({})
+    sid = tl.record({"batch_data": [{"x": 1}]}, note="test")
+    data = tl.get_snapshot_data(sid)
+    assert data == {"batch_data": [{"x": 1}]}
+    assert tl.get_snapshot_data("nonexistent") is None
+
+
+# --- diff_snapshots tests ---
+
+def test_diff_unchanged():
+    batch = [{"sequence_number": 1, "seed": 42}]
+    result = diff_snapshots(batch, batch)
+    assert len(result) == 1
+    assert result[0]["status"] == "unchanged"
+    assert result[0]["changes"] == []
+
+
+def test_diff_changed():
+    old = [{"sequence_number": 1, "seed": 42, "cfg": 1.5}]
+    new = [{"sequence_number": 1, "seed": 99, "cfg": 1.5}]
+    result = diff_snapshots(old, new)
+    assert result[0]["status"] == "changed"
+    assert len(result[0]["changes"]) == 1
+    assert result[0]["changes"][0]["field"] == "seed"
+    assert result[0]["changes"][0]["old"] == 42
+    assert result[0]["changes"][0]["new"] == 99
+
+
+def test_diff_added_and_removed():
+    old = [{"sequence_number": 1, "seed": 1}]
+    new = [{"sequence_number": 2, "seed": 2}]
+    result = diff_snapshots(old, new)
+    assert len(result) == 2
+    statuses = {r["seq_num"]: r["status"] for r in result}
+    assert statuses[1] == "removed"
+    assert statuses[2] == "added"
+
+
+def test_diff_empty():
+    assert diff_snapshots([], []) == []