Add Storm Sewer module: storm-drain network analysis (Rational + Manning + HGL)

Adds a "Storm Sewer" ribbon tab backed by a new in-repo `stormsewer` engine
crate (vendored under crates/). The engine implements standard public-domain
methods — Rational-method peak-flow accumulation, Manning circular-pipe
hydraulics, time-of-concentration / IDF intensity, and an HGL backwater pass
with junction losses — with 24 unit tests and no external dependencies.

Module (src/modules/storm_sewer/):
- Drafting: SS_INLET / SS_JUNCTION / SS_OUTFALL place structure markers and
  SS_PIPE chains pipe runs (interactive CadCommands).
- Analysis: SS_ANALYZE / SS_REPORT / SS_PROFILE open a .ssn network file, run
  the engine, draw the plan and HGL long-section, and print a report.
- SVG icons; SS_* commands registered for command-line autocomplete.

Integration is additive: build.rs auto-discovers the module and one dispatch
block is added to src/app/commands.rs. 7 module integration tests.

See src/modules/storm_sewer/INTEGRATION.md for the design and the optional
follow-up (drive analysis from canvas geometry via XDATA).

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

src/app/commands.rs

@@ -29,6 +29,70 @@ impl OpenCADStudio {
         }
 
         match cmd {
+            // ── Storm Sewer module ──────────────────────────────────────────
+            "SS_ANALYZE" => {
+                use crate::modules::storm_sewer::analysis as ss;
+                match ss::pick_ssn_file() {
+                    Some(read) => match read.and_then(|t| ss::analyze_text(&t)) {
+                        Ok((ents, report)) => {
+                            for e in ents {
+                                let _ = self.tabs[i].scene.add_entity(e);
+                            }
+                            self.tabs[i].scene.bump_geometry();
+                            for line in report.lines() {
+                                self.command_line.push_output(line);
+                            }
+                        }
+                        Err(e) => self.command_line.push_error(&e),
+                    },
+                    None => self.command_line.push_info("Storm sewer: open cancelled."),
+                }
+            }
+            "SS_REPORT" => {
+                use crate::modules::storm_sewer::analysis as ss;
+                match ss::pick_ssn_file() {
+                    Some(read) => match read.and_then(|t| ss::report_text_from(&t)) {
+                        Ok(report) => {
+                            for line in report.lines() {
+                                self.command_line.push_output(line);
+                            }
+                        }
+                        Err(e) => self.command_line.push_error(&e),
+                    },
+                    None => self.command_line.push_info("Storm sewer: open cancelled."),
+                }
+            }
+            "SS_INLET" | "SS_JUNCTION" | "SS_OUTFALL" => {
+                use crate::modules::storm_sewer::structures::PlaceStructure;
+                let cmd = match cmd {
+                    "SS_INLET" => PlaceStructure::inlet(),
+                    "SS_JUNCTION" => PlaceStructure::junction(),
+                    _ => PlaceStructure::outfall(),
+                };
+                self.command_line.push_info(&cmd.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(cmd));
+            }
+            "SS_PIPE" => {
+                let cmd = crate::modules::storm_sewer::structures::PlacePipe::new();
+                self.command_line.push_info(&cmd.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(cmd));
+            }
+            "SS_PROFILE" => {
+                use crate::modules::storm_sewer::analysis as ss;
+                match ss::pick_ssn_file() {
+                    Some(read) => match read.and_then(|t| ss::profile_text(&t)) {
+                        Ok(ents) => {
+                            for e in ents {
+                                let _ = self.tabs[i].scene.add_entity(e);
+                            }
+                            self.tabs[i].scene.bump_geometry();
+                            self.command_line.push_info("Storm sewer HGL profile drawn.");
+                        }
+                        Err(e) => self.command_line.push_error(&e),
+                    },
+                    None => self.command_line.push_info("Storm sewer: open cancelled."),
+                }
+            }
             "NEW" => return Task::done(Message::TabNew),
             "OPEN" => return Task::done(Message::OpenFile),
             "SAVE" | "QSAVE" => return Task::done(Message::SaveFile),