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

server.py

@@ -6662,6 +6662,11 @@ async def pid_line_draw(
     (reported, never refused) and `port_reuse` names ports that already had a
     line (branching is legal). Signal classes get ISA-5.1 markers (`//`, `X`,
     `L`, `o`) as small blocks on segments >= 15 mm; electric is dashed by layer.
+    A signal line carries no pipe line number: it gets no auto-built number
+    and no label, and does not consume the process sequence — a verbatim
+    `line_number` is still written and labelled. With waypoints, a bubble's
+    exit leaves towards the first waypoint (and the entry arrives from the
+    last one); `crossings` follows existing arcs, not their chords.
     """
     from engineering.pid.drawlines import draw_line