Storm Sewer: interactive in-app network — snapping, tracked objects, analyze the drawing

Replaces the .ssn-file workflow with an interactive one driven by the canvas:

- Structures (SS_INLET / SS_JUNCTION / SS_OUTFALL) prompt invert/rim/area/C on
  the command line and commit a CIRCLE tagged with STORMSEWER_STRUCT XDATA
  [kind, invert, rim, area, C]. Outfalls skip area/C.
- SS_PIPE picks a START and END structure (entity-pick), snapping its endpoints
  to their centers, prompts diameter/n, and stores connectivity (the two
  structure handles) in STORMSEWER_PIPE XDATA [diameter, n, from, to].
- SS_ANALYZE / SS_REPORT / SS_PROFILE rebuild the network from the drawn
  entities (data.rs::network_from_entities) and analyze it directly; SS_ANALYZE
  overlays flow/HGL labels without redrawing the geometry.

Because the data lives on the entities as XDATA, the network round-trips to
DWG/DXF. One additive line in src/app/update.rs adds SS_PIPE to the
inject_picked_entity allow-list so the command can read the picked structure's
center for snapping.

15 module tests cover the XDATA round-trip, analyze-from-drawing, and the
command step machines.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/update.rs

@@ -2812,7 +2812,7 @@ impl OpenCADStudio {
                                     .map(|c| {
                                         matches!(
                                             c.name(),
-                                            "DIMTEDIT" | "MLEADERADD" | "MLEADERREMOVE"
+                                            "DIMTEDIT" | "MLEADERADD" | "MLEADERREMOVE" | "SS_PIPE"
                                         )
                                     })
                                     .unwrap_or(false);