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/modules/mod.rs

@@ -137,6 +137,7 @@ pub mod insert;
 pub mod model;
 pub mod layout;
 pub mod manage;
+pub mod storm_sewer;
 pub mod view;
 
 // ── Auto-generated module registry ───────────────────────────────────────