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/draw.py

@@ -21,6 +21,7 @@ from engineering.understand.takeoff import (
     check_pipe_options,
     check_section_rules,
     pipe_rows,
+    unit_of,
 )
 
 __all__ = ["run_cable_takeoff", "run_pipe_takeoff"]
@@ -49,7 +50,7 @@ async def run_pipe_takeoff(
     length_source: str = "auto",
     force: bool = False,
     layers=None,
-    tol: float = 1.0,
+    tol: float | None = None,
 ) -> dict:
     """`pipe_rows` over a P&ID and an optional layout, plus the workbook when `output` is named.
 
@@ -64,14 +65,17 @@ async def run_pipe_takeoff(
         vertical_allowance=vertical_allowance,
         length_source=length_source,
     )
-    tol = _check_tol(tol)
+    tol = None if tol is None else _check_tol(tol)
     if length_source == "layout" and not layout_path:
         raise ValueError("length_source='layout' needs a layout drawing; none was given")
     pid = await take_snapshot(backend, pid_path)
     layout = await take_snapshot(backend, layout_path) if layout_path else None
 
     def compute() -> dict:
-        network = build_network(pid, layers=layers, tol=tol)
+        # 1 mm in the unit the P&ID's geometry implies: a fixed 1.0 would be a
+        # metre on a metre drawing, merging ends a metre apart and dropping pipes
+        joint = tol if tol is not None else 1.0 / (unit_of(pid)["m_per_unit"] * 1000.0)
+        network = build_network(pid, layers=layers, tol=joint)
         return pipe_rows(
             network,
             pid,