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

discovery/aliases.py

@@ -1384,6 +1384,8 @@ TOOL_ALIASES: dict[str, ToolAliases] = {
             "line number",
             "connect the pump to the vessel",
             "pneumatic signal",
+            "unnumbered signal line",
+            "route a line through waypoints",
         ),
     ),
     "pid_graph": ToolAliases(