feat(plugin): InteractiveCommand hook for click-to-place (API v2)

Plugins could dispatch commands but not register interactive (click-to-
place) tools — the gap mf4633 flagged on #100 for Storm Sewer's SS_INLET
/ SS_PIPE. Add an InteractiveCommand trait + CommandStep to ocs_plugin_api
and HostApi::start_interactive; a host adapter bridges it to the internal
CadCommand, so a plugin tool drives the host's point-collection flow.

Hybrid by construction: the same command works by clicking in the viewport
AND by feeding coordinates over --serve (run "CMD x,y x,y"). Adding a
HostApi method changes the contract vtable, so API_VERSION bumps to 2 —
v1 plugin binaries are now refused at load.

Part of the #100 extensibility epic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/plugin-architecture.md

@@ -203,7 +203,7 @@ version = "0.1.0"
 description = "…"
 
 [opencad]
-api_version = 1
+api_version = 2
 ribbon_order = 50
 command_prefixes = ["EX_"]
 xdata_apps = []
@@ -225,6 +225,24 @@ tool fires `ModuleEvent::Command("EX_FOO")`, which round-trips to
 tool request a native file picker; on selection the host dispatches
 `"<command> <path>"` back with the path's original case preserved.
 
+### Interactive (click-to-place) commands
+
+For tools that collect points — placing a structure, drawing a pipe — call
+`host.start_interactive(Box::new(my_cmd))` from `dispatch`, where `my_cmd`
+implements `ocs_plugin_api::host::InteractiveCommand`:
+
+```rust
+fn on_point(&mut self, pt: [f64; 3]) -> CommandStep {
+    // … return NeedPoint, Commit(entity), CommitAndEnd(entity), Done, or Cancel
+}
+```
+
+The host drives it through its normal point-collection flow, so the **same**
+command works by clicking in the viewport and by feeding coordinates over the
+`--serve` automation API (`run "MY_CMD 0,0 10,10"`). This is what bumped the
+API to **v2** — the added `HostApi` method changes the contract's vtable, so v1
+binaries are refused at load.
+
 ### XDATA — domain persistence
 
 Store domain data on entities as XDATA (under your `xdata_apps` ids), not in a

docs/plugin-template/plugin.toml

@@ -7,7 +7,7 @@ author = "Your Name"
 license = "GPL-3.0-only"
 
 [opencad]
-api_version = 1
+api_version = 2
 ribbon_order = 60
 command_prefixes = ["MP_"]
 xdata_apps = ["MYPLUGIN_RECORD"]