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>
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