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>

Cargo.toml

@@ -4,6 +4,9 @@ version = "0.5.0"
 edition = "2021"
 build = "build.rs"
 
+[workspace]
+members = ["crates/stormsewer"]
+
 [dependencies]
 # vtkio (pulled in transitively via truck-meshalgo) depends on xz2 → lzma-sys,
 # which by default links the system liblzma dynamically. On macOS that bakes a
@@ -31,6 +34,9 @@ truck-shapeops = "0.4"
 # Fast non-cryptographic hasher. Handle/u64 keys dominate the hot block,
 # hatch, draw-depth and wire caches; the default SipHash is overkill there.
 rustc-hash = "2"
+# Storm-sewer hydrology/hydraulics engine (Rational + Manning + HGL). Local
+# crate; powers the `storm_sewer` ribbon module.
+stormsewer = { path = "crates/stormsewer" }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 windows-sys = { version = "0.61", features = ["Win32_UI_Shell", "Win32_UI_WindowsAndMessaging"] }