fix(task-14): foreign-line membership before junctions, and edges measure the arc the drawing holds

Two reader defects, both about trusting a derived number over the drawing.

Stale junctions: endpoints were resolved before a foreign line's membership
was decided, so a P&ID line ending on a foreign line that was then discarded
(it touched no node) kept `{"junction": "J1"}` while `junctions` was empty
and `stats.dangling` was one short. Node attachment now runs first for every
edge, the foreign lines that attach are admitted, and only then are junctions
and dangling ends resolved over the edges actually in the graph.

Chord length: `length` was a chord walk over the vertices, 12.5% short on a
semicircular crossing jump with no field saying so, and junction detection
tested the chord too, so a line ending on the arc dangled. Both engines now
report per-vertex `bulges` next to `points` (COM asks GetBulge only when
`Length` exceeds the chord walk, since an arc is always longer than its
chord); the reader takes the engine's bulge-aware `length` and tests
endpoints against the arc. The chord sum survives only for an engine that
reports no length, and is then labelled `length_approximate`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

tests/test_pid_graph.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import math
+
 import pytest
 
 from engineering.pid.drawlines import draw_line
@@ -181,3 +183,116 @@ async def test_geometry_is_optional(backend):
     assert "vertices" not in lean["edges"][0]
     full = await build_graph(backend, include_geometry=True)
     assert len(full["edges"][0]["vertices"]) >= 2
+
+
+async def test_a_line_ending_on_a_discarded_foreign_line_dangles(backend):
+    """A foreign line is an edge only when an end touches a node (spec §9.2 4c).
+
+    Junctions used to be resolved before that decision, so a P&ID line ending
+    on a foreign line that was then dropped kept ``{"junction": "J1"}`` while
+    ``junctions`` was empty and ``stats.dangling`` was one short — an id
+    Task 15's line list would dereference into nothing."""
+    main = (await backend.entity_create_line(0, 0, 50, 0, layer="PROCESS-PIPING-MAIN")).handle
+    await backend.entity_create_line(50, -20, 50, 20, layer="0")
+    graph = await build_graph(backend)
+    edge = graph["edges"][0]
+    assert edge["id"] == main and edge["from"] is None and edge["to"] is None
+    assert graph["junctions"] == []
+    assert {(d["edge"], d["end"]) for d in graph["dangling"]} == {(main, "from"), (main, "to")}
+    assert graph["stats"]["dangling"] == 2
+
+
+async def test_a_foreign_line_ending_on_another_discarded_foreign_line_dangles(backend):
+    blk = backend._doc.blocks.new(name="GATE_VALVE")
+    blk.add_lwpolyline([(-4, -2), (-4, 2), (0, 0)], close=True)
+    blk.add_lwpolyline([(4, -2), (4, 2), (0, 0)], close=True)
+    valve = (await backend.block_insert("GATE_VALVE", 50, 50)).handle
+    kept = (await backend.entity_create_line(46, 50, 10, 50, layer="0")).handle  # on the bbox
+    await backend.entity_create_line(10, 50, 10, 0, layer="0")  # touches nothing: not an edge
+    graph = await build_graph(backend)
+    assert [e["id"] for e in graph["edges"]] == [kept]
+    edge = graph["edges"][0]
+    assert edge["from"] == {"node": valve, "port": "p1"} and edge["to"] is None
+    assert graph["junctions"] == []
+    assert [(d["edge"], d["end"]) for d in graph["dangling"]] == [(kept, "to")]
+    assert graph["stats"]["dangling"] == 1
+
+
+async def test_edge_length_is_the_engines_measurement_not_a_chord_walk(backend):
+    """A curved crossing "jump" is an arc in the drawing; the chord walk over
+    its vertices was 5.7 units short and said nothing about it. The engine
+    measures the real geometry, and a line ending *on the arc* is a junction."""
+    msp = backend._doc.modelspace()
+    pipe = msp.add_lwpolyline(
+        [(-20, 0, 0), (0, 0, 1), (10, 0, 0), (30, 0, 0)],
+        format="xyb",
+        dxfattribs={"layer": "PROCESS-PIPING-MAIN"},
+    )
+    pipe = pipe.dxf.handle
+    measured = (await backend.entity_get(pipe)).properties["length"]
+    assert measured == pytest.approx(20 + 5 * math.pi + 20)
+    # bulge 1 from (0,0) to (10,0) bows to -y: centre (5,0), radius 5 → apex (5,-5)
+    branch = await backend.entity_create_line(5, -5, 5, -30, layer="PROCESS-PIPING-SECONDARY")
+    branch = branch.handle
+    graph = await build_graph(backend)
+    edges = {e["id"]: e for e in graph["edges"]}
+    assert edges[pipe]["length"] == pytest.approx(measured)
+    assert "length_approximate" not in edges[pipe]
+    assert edges[branch]["from"] and "junction" in edges[branch]["from"]
+    junction = graph["junctions"][0]
+    assert (junction["x"], junction["y"]) == pytest.approx((5.0, -5.0))
+    assert set(junction["edges"]) == {pipe, branch}
+    full = await build_graph(backend, include_geometry=True)
+    assert {e["id"]: e for e in full["edges"]}[pipe]["bulges"] == [0.0, 1.0, 0.0, 0.0]
+
+
+def test_point_to_arc_distance_follows_the_bulge():
+    from engineering.pid.graph import _point_segment_distance
+
+    a, b = (0.0, 0.0), (10.0, 0.0)
+    assert _point_segment_distance((5, -5), a, b, 1.0) == pytest.approx(0.0)  # CCW apex
+    assert _point_segment_distance((5, 5), a, b, -1.0) == pytest.approx(0.0)  # CW apex
+    assert _point_segment_distance((5, 5), a, b, 1.0) == pytest.approx(math.hypot(5, 5))
+    assert _point_segment_distance((5, -2.5), a, b, 0.5) == pytest.approx(0.0)  # sagitta
+    assert _point_segment_distance((5, 2.5), a, b, 0.5) == pytest.approx(5.0)
+    assert _point_segment_distance((5, 0), a, b, 0.0) == pytest.approx(0.0)  # straight
+
+
+def test_com_polyline_bulges_are_read_only_when_length_says_an_arc_exists():
+    """The COM engine pays one round trip per vertex for GetBulge, so it asks
+    only when ActiveX ``Length`` exceeds the chord walk — an arc is always
+    longer than its chord, so equality proves every bulge is zero."""
+    from types import SimpleNamespace
+
+    from backends.com_backend import _entity_info
+
+    def fake(coords, length, bulges):
+        calls = []
+
+        def get_bulge(i):
+            calls.append(i)
+            return bulges[i]
+
+        ent = SimpleNamespace(
+            ObjectName="AcDbPolyline",
+            Coordinates=coords,
+            Normal=(0.0, 0.0, 1.0),
+            Elevation=0.0,
+            Handle="2F",
+            Layer="0",
+            Color=256,
+            Linetype="ByLayer",
+            Visible=True,
+            Closed=False,
+            Length=length,
+            GetBoundingBox=lambda: ((0.0, 0.0, 0.0), (1.0, 1.0, 0.0)),
+            GetBulge=get_bulge,
+        )
+        return ent, calls
+
+    straight, calls = fake((0.0, 0.0, 10.0, 0.0, 10.0, 10.0), 20.0, [0.0, 0.0, 0.0])
+    assert _entity_info(straight).properties["bulges"] == [0.0, 0.0, 0.0]
+    assert calls == []
+    curved, calls = fake((0.0, 0.0, 10.0, 0.0, 10.0, 10.0), 10 + 5 * math.pi, [0.0, 1.0, 0.0])
+    assert _entity_info(curved).properties["bulges"] == [0.0, 1.0, 0.0]
+    assert calls == [0, 1, 2]