fix(pid): signal lines carry no fabricated number; a bubble exits towards the first waypoint

An ISA-5.1 signal line has no pipe line number, yet every pneumatic /
electric line got the bare sequence as its number and a label, and ate a
process sequence number. Signal classes now leave line_number None (a
verbatim one is still honoured), seq None, no label. A radial port aimed
at the far end even with waypoints, so the first segment cut through the
bubble; aim_points (lines.py) aims at the first/last waypoint for both
draw_line and the dry run.

Merge note: benchmarks/adapters/autocad_mcp_pro.py (final group's file)
had its two _task_pid_roundtrip assertion lines moved to the new truth —
the signal row is unnumbered, the connector row is 3.

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

engineering/pid/lines.py

@@ -198,6 +198,21 @@ def route(start, start_dir, end, end_dir, stub: float = 5.0, mode="auto") -> lis
     return valid[0]
 
 
+def aim_points(start, end, mode) -> tuple[Point, Point]:
+    """What each end of a line aims at: the other end, or — with waypoints —
+    the first waypoint for the start and the last one for the end.
+
+    A radial (bubble) port leaves its circle along the axis towards its aim
+    point. Aiming at the far end while the route is a waypoint list put the
+    exit on the wrong side of the bubble, so the first segment cut through it.
+    """
+    if isinstance(mode, (list, tuple)) and len(mode) > 0:
+        return _finite_point("waypoints[0]", mode[0]), _finite_point(
+            f"waypoints[{len(mode) - 1}]", mode[-1]
+        )
+    return _finite_point("end", end), _finite_point("start", start)
+
+
 def label_placement(vertices, offset: float = 1.5) -> tuple[float, float, float, int]:
     """Midpoint of the longest segment, offset to its left, readable rotation."""
     pts = [(float(x), float(y)) for x, y in vertices]