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>

crates/ocs_plugin_api/src/manifest.rs

@@ -1,8 +1,9 @@
 //! Plugin identity and capability declaration.
 
 /// Host plugin API version. Bump when the host runtime surface breaks
-/// compatibility.
-pub const API_VERSION: u32 = 1;
+/// compatibility. v2 added `HostApi::start_interactive` (the
+/// `InteractiveCommand` hook) — a vtable change, so v1 binaries are refused.
+pub const API_VERSION: u32 = 2;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct ApiVersion {