rename crates/ → vessels/ — El's word for buildable units

Per the consolidation onto El: 'crates' is the Rust word, 'vessel' is
El's (per spec/language.md §15). The directory rename is the structural
marker that this slot holds an El buildable unit, even if its current
contents are still Rust pending port.

Mechanical: git mv crates vessels, sed workspace members and any path
dependencies, update CI workflow paths, update README references.
Cross-repo path dependencies (`../foo/crates/bar`) updated workspace-
wide so cargo metadata still resolves where the Rust still builds.

ide/vessels/el-plugin-host/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "el-plugin-host"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+serde       = { workspace = true }
+serde_json  = { workspace = true }
+thiserror   = { workspace = true }