Add plugin host architecture and Storm Sewer add-on package

Generic plugin runtime (src/plugin/, HostSession, per-tab plugin_state, command router). QGIS-style add-on layout with plugin.toml and single PluginRegistration. Storm Sewer refactored as reference consumer with dispatch.rs, catchments, LandXML, headless tests (43 passing). Core update.rs decoupled from storm_sewer via CadCommand object-pick hooks. Docs: plugin-architecture.md, plugin-template, PR and issue-78 reply drafts.

docs/plugin-template/README.md

@@ -0,0 +1,31 @@
+# Add-on plugin template
+
+Copy this folder to `src/modules/<your_module>/` and rename placeholders.
+
+## Quick start
+
+1. Copy files into `src/modules/my_plugin/`.
+2. Replace `MY_PLUGIN`, `my_plugin`, `opencad.my_plugin`, `MP_` throughout.
+3. Add `pub mod my_plugin;` to `src/modules/mod.rs`.
+4. `cargo build` — ribbon tab and commands register automatically.
+
+## Required files
+
+| File | Purpose |
+|------|---------|
+| `plugin.toml` | Metadata (QGIS-style); excludes dir from `build.rs` ribbon scan |
+| `manifest.rs` | Compile-time `PluginManifest` — keep in sync with `plugin.toml` |
+| `register.rs` | `inventory::submit!(PluginRegistration { … })` only |
+| `plugin.rs` | Thin `BuiltinPlugin` impl |
+| `dispatch.rs` | All command handlers |
+| `mod.rs` | `CadModule` ribbon + `CommandRegistration` for autocomplete |
+| `PLUGIN.md` | XDATA schemas and command reference |
+
+## Optional
+
+| File | Purpose |
+|------|---------|
+| `state.rs` | Per-document tab state via `host.ensure_plugin_state(PLUGIN_ID, …)` |
+| `crates/my_engine/` | Headless domain logic (no iced/acadrust) |
+
+See `docs/plugin-architecture.md` for the full spec.