fix(task-6): the reader no longer hands a signal line the neighbouring pipe's label

pid_graph step 5 ran the TEXT label fallback for every edge without a
number. Once signal lines stopped carrying a fabricated number (3c0babf),
every pneumatic/electric run drawn by pid_line_draw entered it, and a
transmitter-to-controller run 12 mm above a labelled header came back as
line_number="100-P-1", number_source="label" — on both engines, with no
confidence field on edges to flag it.

The fallback now skips an edge that carries its own record (an
ACADMCP_PID payload: its `number: null` is the answer) and any
signal-class edge, payload or layer-classified (`signal_unknown`
included): LINE_NUMBER_RE is the pipe grammar, so a match near a signal
line always belongs to a neighbouring pipe. A foreign process line still
takes its label; a verbatim number on a signal line still reads back
from XDATA.

Tests: the finding's layout in test_pid_graph (engine-drawn signal beside
a labelled pipe, verbatim signal number, foreign electric and
signal_unknown lines beside a labelled foreign pipe) and the line_list
row in test_pid_deliverables — three of the four fail on the old reader.

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

engineering/pid/graph.py

@@ -12,7 +12,7 @@ import re
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING
 
-from .lines import LAYER_TO_CLASS, PID_LINE_LAYERS
+from .lines import LAYER_TO_CLASS, LINE_CLASSES, PID_LINE_LAYERS
 from .symbols import Port, transform_port
 from .tags import parse_tag
 from .xdata import read_payload
@@ -55,6 +55,15 @@ CONFIDENCE = {"catalog": 1.0, "xdata": 0.95, "heuristic": 0.6, "inferred": 0.3}
 _PAGE = 1000
 
 
+def _is_signal_class(line_class: str | None) -> bool:
+    """True for a class that is a signal in the ISA-5.1 sense (`signal_unknown`
+    is the layer-classified one that LINE_CLASSES does not list)."""
+    if line_class == "signal_unknown":
+        return True
+    cls = LINE_CLASSES.get(line_class) if line_class else None
+    return bool(cls and cls.kind == "signal")
+
+
 @dataclass
 class _Node:
     id: str
@@ -684,7 +693,15 @@ async def build_graph(
             if found:
                 node.tag, node.tag_source = found, "text"
     for edge in edges.values():
-        if edge.line_number:
+        if edge.line_number or edge.payload or _is_signal_class(edge.line_class):
+            # The label search is a fallback for a line that carries no
+            # record of its own. An engine-authored line (payload) is that
+            # record — its `number: null` is the answer, not an invitation to
+            # adopt the nearest pipe's label. A signal line never has a pipe
+            # line number (ISA-5.1), and LINE_NUMBER_RE is the pipe grammar,
+            # so any match near it belongs to a neighbouring pipe: the
+            # ordinary transmitter-to-controller run 12 mm above a header
+            # would otherwise be listed under the header's number.
             continue
         a, b = max(zip(edge.vertices, edge.vertices[1:], strict=False), key=lambda s: math.dist(*s))
         found = _text_near(