Document conductor segments and the non-finite value check (PR #1161)

conductors.md

@@ -199,6 +199,18 @@ matches where its terminals are — because something moved without the path
 being updated — QElectroTech quietly discards it on load and re-routes
 automatically. The drawing looks fine; the hand-routing is lost.
 
+> **Status: pending.** The next paragraph describes
+> [PR #1161](https://github.com/qelectrotech/qelectrotech-source-mirror/pull/1161),
+> not yet merged. Until it lands, one unreadable length (`nan`) in a damaged
+> file turns the whole stored route into `nan` on the first save, and the
+> next save drops it. This notice will come out once it merges.
+
+The same happens when a stored segment length is not a usable number — a
+project file damaged by an outside tool or a disk error, with `nan` or `inf`
+where a length should be. The conductor is routed automatically on load and
+the file saves normally from then on. The storage format is described in
+**[conductors (XML)](project_diagram_child_conductors)**.
+
 ---
 
 ## 7. Colouring quickly

data_files/project_diagram_child_conductors.md

@@ -14,9 +14,33 @@ attribute : none
    * terminal1, terminal2 -- the terminals joined
    * terminalname1, terminalname2 -- their names
    * element1_name, element2_name, element1_label, element2_label -- cached labels
-   * x, y, userx, usery, rotation -- geometry of the conductor text
+   * x, y -- the conductor's own position
+   * userx, usery -- the label's position, written only when the label has
+     been moved by hand
+   * rotation -- the label's rotation, written only when it has been rotated
+     by hand
    * freezeLabel -- `true` / `false`
 
+   A conductor whose route was shaped by hand also has one `<segment>` child
+   per segment, in order from the first terminal:
+   * orientation -- `horizontal` or `vertical`
+   * length -- signed length in pixels; negative goes left or up
+
+   On load the lengths must add up to the distance between the two terminals
+   (within one pixel), or the stored route is discarded and the conductor is
+   routed automatically. A conductor with no `<segment>` children is routed
+   automatically.
+
+> **Status: pending.** The next paragraph describes
+> [PR #1161](https://github.com/qelectrotech/qelectrotech-source-mirror/pull/1161),
+> not yet merged. This notice will come out once it merges.
+
+Every number above must be finite. `nan`, `inf` and `-inf` parse as numbers,
+so QElectroTech checks for them: a non-finite segment length is ignored (so
+the route no longer adds up and is routed automatically), a non-finite `x` or
+`y` is read as 0, and a non-finite `userx`, `usery` or `rotation` is ignored
+as if the label had never been moved or rotated.
+
 Conductors are written in a stable sorted order derived from both endpoints, so
 that saving twice produces byte-identical files.