fix(understand): the pipe network's tolerance follows the P&ID's unit, and a dropped stub is said

pipe_takeoff merged line ends closer than tol = 1.0 drawing units and
silently dropped any segment shorter than that. On a millimetre P&ID that is
1 mm. On a metre drawing it is a metre: ends a metre apart were merged and
every pipe under a metre disappeared from the totals, with no warning. The
review of the network engine found this.

- tol now defaults to 1 mm in the unit the P&ID's geometry implies: 1.0 on
  a millimetre drawing, 0.001 on a metre one. A tol the caller gives is
  used as given.
- build_network counts the segments that collapse to a point
  (stats.segments_below_tol), and pipe_rows turns a non-zero count into a
  warning naming the tolerance.

A new wire test builds a metre P&ID with a 0.6 m pipe and a 5 m pipe. Before
this change the takeoff said 5.0 m; now it says 5.6 m. The network test
checks that a sub-tolerance stub is counted. Registry, packs, aliases and
every track H suite: 472 passed. The inventory matches the live surface.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

tests/test_understand_network.py

@@ -103,6 +103,14 @@ def test_ends_within_tol_merge_into_one_run():
     assert run.ends == ((0.0, 0.0), (2000.0, 0.0))
 
 
+def test_a_segment_shorter_than_tol_is_counted_when_it_is_dropped():
+    # a 0.5-unit stub collapses to one vertex at tol 1.0: gone from the network,
+    # but never silently - stats say how many went
+    result = net(line("1", (0.0, 0.0), (1000.0, 0.0)), line("2", (2000.0, 0.0), (2000.5, 0.0)))
+    assert result["stats"]["segments_below_tol"] == 1
+    assert net(line("1", (0.0, 0.0), (1000.0, 0.0)))["stats"]["segments_below_tol"] == 0
+
+
 def test_a_branch_ending_on_a_header_is_a_t_junction():
     result = net(line("1", (0.0, 0.0), (2000.0, 0.0)), line("2", (1000.0, 0.0), (1000.0, 800.0)))
     (run,) = result["runs"]