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

engineering/understand/network.py

@@ -539,10 +539,13 @@ def build_network(snap: Snapshot, *, layers=None, tol=1.0, label_search=None) ->
     raw = _segments(snap, chosen)
     verts = _Vertices(tol)
     snapped = []
+    below_tol = 0  # segments whose two ends merge into one vertex: dropped, and counted
     for a, b, bulge, handle, layer in raw:
         u, v = verts.index(a), verts.index(b)
         if u != v:
             snapped.append((u, v, bulge, handle, layer))
+        else:
+            below_tol += 1
 
     # equipment first: an INSERT a tag labels is never a fitting
     occurrences = tag_occurrences(snap)
@@ -888,6 +891,7 @@ def build_network(snap: Snapshot, *, layers=None, tol=1.0, label_search=None) ->
             "tol": tol,
             "label_search": search,
             "segments": len(raw),
+            "segments_below_tol": below_tol,
             "pieces": len(pieces),
             "overlap_removed": overlap_count,
             "overlap_length": overlap_length,