feat(plugin): lift host surface behind a HostApi trait (phase 1b)

Plugins targeted the host's concrete HostSession type. Add a HostApi
trait in ocs_plugin_api behind an optional `host` feature (the only thing
that pulls acadrust, so the core crate stays dependency-free). HostSession
implements it and BuiltinPlugin::dispatch now takes `&mut dyn HostApi`, so
an out-of-tree add-on compiles against the contract crate alone. Per-tab
plugin state is reached through object-safe plugin_state* helpers.

Completes the phase-1 host-surface extraction in the #100 epic.

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

Cargo.toml

@@ -24,7 +24,7 @@ solid3d = ["dep:truck-meshalgo", "dep:truck-shapeops", "dep:lzma-sys"]
 [dependencies]
 # Stable, dependency-free add-on contract (manifest + ribbon/CadModule types).
 # Plugin authors target this crate's semver, not OpenCADStudio internals.
-ocs_plugin_api = { path = "crates/ocs_plugin_api" }
+ocs_plugin_api = { path = "crates/ocs_plugin_api", features = ["host"] }
 # 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
 # Homebrew dylib path (/opt/homebrew/.../liblzma.5.dylib) into the binary, so