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/takeoff.py

@@ -802,6 +802,13 @@ def pipe_rows(
         )
     }
     warnings = [u["warning"] for u in (pid_unit, layout_unit) if u and u["warning"]]
+    dropped = int(network.get("stats", {}).get("segments_below_tol") or 0)
+    if dropped:
+        warnings.append(
+            f"{dropped} P&ID segment(s) shorter than the junction tolerance "
+            f"({network['stats']['tol']:g} drawing units) collapsed to a point and are not "
+            "counted"
+        )
     if not scale_verified:
         warnings.append(
             "scale_verified: false - lengths are drawn lengths on a P&ID that no scale check "