Server-side trails: the current run and one previous

The integration now records the path itself (trails.py): it watches the
source entity's state changes, so recording needs no open card, has no
multi-tab write races, and every screen sees the same line — reloads
included, which retires the localStorage snapshot after one day of
life. Stored per marker: the current run plus exactly one previous
(owner call — users want cleaned-vs-uncleaned at a glance). The
previous run renders at 40% opacity; the current one still trims its
live tail so it never outruns the puck. Runs rotate on start or map
switch, points cap at 2000 with decimation, store writes debounce 10 s,
and houseplan_trail_updated pushes live cards. TrailBook is pure under
5 backend tests; the WS command degrades silently on older backends.

tests_backend/test_trails.py

@@ -0,0 +1,66 @@
+"""TrailBook: the pure part of the server-side vacuum trails."""
+import sys, pathlib
+sys.path.insert(0, str(pathlib.Path(__file__).parent.parent / "custom_components" / "houseplan"))
+import importlib.util
+spec = importlib.util.spec_from_file_location(
+    "trailbook_pure",
+    pathlib.Path(__file__).parent.parent / "custom_components" / "houseplan" / "trails.py",
+)
+# import only TrailBook without HA deps: read the source and exec the class
+src = (pathlib.Path(__file__).parent.parent / "custom_components" / "houseplan" / "trails.py").read_text(encoding="utf-8")
+ns = {"Any": object, "annotations": None}
+exec(src[src.index("TRAIL_CAP"):src.index("class TrailRecorder")], ns)
+TrailBook = ns["TrailBook"]
+TRAIL_CAP = ns["TRAIL_CAP"]
+
+
+def test_append_dedups_and_records():
+    b = TrailBook()
+    assert b.on_point("m", "0", 1.0, 2.0, 100.0)
+    assert not b.on_point("m", "0", 1.0, 2.0, 101.0)  # same point
+    assert b.on_point("m", "0", 3.0, 4.0, 102.0)
+    run = b.data["m"]["current"]
+    assert run["points"] == [[1.0, 2.0], [3.0, 4.0]]
+    assert run["ended"] is None
+
+
+def test_end_then_new_run_rotates_current_to_previous():
+    b = TrailBook()
+    b.on_point("m", "0", 1, 1, 1.0)
+    b.on_point("m", "0", 2, 2, 2.0)
+    assert b.end_run("m", 3.0)
+    assert not b.end_run("m", 4.0)  # idempotent
+    b.on_point("m", "0", 9, 9, 5.0)
+    rec = b.data["m"]
+    assert rec["previous"]["points"] == [[1, 1], [2, 2]]
+    assert rec["previous"]["ended"] == 3.0
+    assert rec["current"]["points"] == [[9, 9]]
+    # a third run forgets the first entirely — exactly two are kept
+    b.end_run("m", 6.0)
+    b.on_point("m", "0", 7, 7, 7.0)
+    assert rec if rec is b.data["m"] else True
+    assert b.data["m"]["previous"]["points"] == [[9, 9]]
+
+
+def test_map_switch_mid_run_starts_a_new_run():
+    b = TrailBook()
+    b.on_point("m", "floor1", 1, 1, 1.0)
+    b.on_point("m", "floor2", 2, 2, 2.0)
+    assert b.data["m"]["current"]["map_id"] == "floor2"
+    assert b.data["m"]["previous"]["map_id"] == "floor1"
+
+
+def test_cap_decimates_but_keeps_the_freshest_point():
+    b = TrailBook()
+    for i in range(TRAIL_CAP + 1):
+        b.on_point("m", "0", float(i), 0.0, float(i))
+    pts = b.data["m"]["current"]["points"]
+    assert len(pts) <= TRAIL_CAP // 2 + 2
+    assert pts[-1] == [float(TRAIL_CAP), 0.0]
+
+
+def test_junk_store_data_tolerated():
+    b = TrailBook("not a dict")
+    assert b.data == {}
+    b.on_point("m", "0", 1, 1, 1.0)
+    assert b.data["m"]["current"]["points"] == [[1, 1]]