refactor(understand): one reading of units, plan copies, scale and rooms

Groups U and N were built side by side, so the takeoffs carried narrow
stand-ins for what group U reads properly. On main they read through U:

- describe.drawing_units is the one call every reader makes (kept records,
  labelled room faces as evidence); describe, scale_check and the takeoffs
  all use it. Without the room faces, a layout that declares inches over
  millimetres can tie three ways and fall to the header.
- layout_tags takes its plan copies from describe.prepare, room_regions its
  rooms from describe.find_rooms, and pipe_rows its scale from scale_check,
  per-room groups included. scale_stand_in and the takeoff's own unit bands
  and cluster grid are gone.
- test_understand_one_reading pins the agreement: one unit, one plan copy,
  one verdict and one set of rooms per drawing, across drawing_understand,
  drawing_scale_check and the two takeoffs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

tests/test_understand_one_reading.py

@@ -0,0 +1,132 @@
+"""One reading of a drawing: the takeoffs see what drawing_understand and drawing_scale_check see.
+
+Groups U and N were built side by side from one base, so Task 6 carried narrow
+stand-ins: its own unit bands, a 2 % grid of plan copies, a scale statistic
+without room groups, its own room faces. On main the takeoffs read units, plan
+copies, scale and rooms through group U's readers, so the four tools never
+give one drawing two units, two plan copies, two verdicts or two sets of rooms.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from engineering.understand import takeoff
+from engineering.understand.describe import describe, drawing_units, prepare
+from engineering.understand.network import build_network
+from engineering.understand.scale import scale_check
+from engineering.understand.snapshot import Snapshot, read_snapshot
+from tests.fixtures.plant_pair import build_plant_pair
+
+
+@pytest.fixture(scope="module")
+def plant(tmp_path_factory):
+    truth = build_plant_pair(tmp_path_factory.mktemp("one_reading"))
+    return truth, read_snapshot(truth["pid"]), read_snapshot(truth["layout"])
+
+
+def empty(insunits):
+    return Snapshot(
+        source="empty.dxf",
+        insunits=insunits,
+        extmin=None,
+        extmax=None,
+        layers={},
+        layouts=(),
+        records=(),
+    )
+
+
+def test_one_unit_per_drawing(plant):
+    _truth, pid, layout = plant
+    check = scale_check(pid, layout)
+    for snap, side in ((pid, "a"), (layout, "b")):
+        unit = takeoff.unit_of(snap)
+        assert unit["inferred"] == describe(snap)["units"]["inferred"]
+        assert unit["inferred"] == check["units"][side]["unit"]
+    assert takeoff.unit_of(layout)["inferred"] == "mm"  # INSUNITS says inches: Review Focus 5
+
+
+def test_the_takeoff_unit_carries_drawing_units_evidence_and_warning(plant):
+    _truth, pid, layout = plant
+    for snap in (pid, layout):
+        mine, theirs = takeoff.unit_of(snap), drawing_units(snap)
+        assert mine["declared"] == theirs["declared"]["name"]
+        assert mine["m_per_unit"] == pytest.approx(theirs["mm_per_unit"] / 1000.0)
+        assert mine["evidence"] == theirs["evidence"]
+        expected = f"{snap.source}: {theirs['warning']}" if theirs["warning"] else None
+        assert mine["warning"] == expected
+
+
+def test_a_unit_nothing_measures_keeps_the_declared_one():
+    unit = takeoff.unit_of(empty(5))
+    assert (unit["declared"], unit["inferred"], unit["m_per_unit"], unit["warning"]) == (
+        "cm",
+        "cm",
+        0.01,
+        None,
+    )
+
+
+def test_a_unitless_drawing_nothing_measures_is_read_in_mm_and_says_so():
+    unit = takeoff.unit_of(empty(0))
+    assert (unit["declared"], unit["inferred"], unit["m_per_unit"]) == (None, "mm", 0.001)
+    assert unit["warning"] == (
+        "empty.dxf: INSUNITS declares Unitless (0) and no measurement settles the unit; "
+        "the takeoff reads it as mm."
+    )
+
+
+def test_plan_copies_are_the_clusters_drawing_understand_reports(plant):
+    truth, _pid, layout = plant
+    found = takeoff.layout_tags(layout, truth["tag_positions"]["layout"])
+    boxes = [tuple(cluster["box"]) for cluster in prepare(layout)["clusters"]]
+    assert found["clusters"] == len(boxes) == truth["cluster_count"]
+    assert tuple(found["cluster"]) in boxes
+
+
+def test_the_takeoff_scale_is_drawing_scale_check(plant):
+    _truth, pid, layout = plant
+    result = takeoff.pipe_rows(build_network(pid), pid, layout)
+    check = scale_check(pid, layout)
+    for key in ("verdict", "factor", "median", "within_10pct", "pair_count", "groups"):
+        assert result["scale"][key] == check[key], key
+
+
+def test_rooms_are_the_rooms_drawing_understand_reports(plant):
+    truth, _pid, layout = plant
+    cluster = takeoff.layout_tags(layout, truth["tag_positions"]["layout"])["cluster"]
+    regions = takeoff.room_regions(layout, cluster)
+    assert sorted(key for key, _face in regions["faces"]) == sorted(
+        str(room["number"]) for room in truth["rooms"]
+    )
+    reported = {
+        (str(row["number"] or row["name"]), round(row["face"]["area"], 3))
+        for row in describe(layout)["rooms"]
+        if row["face"] is not None
+    }
+    for key, face in regions["faces"]:
+        assert (key, round(face.area, 3)) in reported
+
+
+def test_no_second_reader_is_left():
+    for name in (
+        "scale_stand_in",
+        "INSUNITS_NAMES",
+        "TEXT_BAND_M",
+        "SPAN_BAND_M",
+        "TYPICAL_SPAN_M",
+        "TYPICAL_TEXT_M",
+        "CLUSTER_GAP_SHARE",
+        "MIN_PAIR_MM",
+        "MIN_TAGS",
+        "SCALE_BAND",
+        "TO_SCALE_SHARE",
+        "SCHEMATIC_SHARE",
+        "_model_points",
+        "_robust_box",
+        "_text_heights",
+        "_mark_segment",
+        "_clusters",
+    ):
+        assert not hasattr(takeoff, name), name