Plug-ins: doc comments for the functions the filter changes touch

The docstring check of the review counts the functions a pull request
touches: effect.apply, effect.setParams, plugin.run, the line cap
name, the reader of an output object and seven of the object tests had
no doc comment. Each now says what it does or checks; the join and cap
names get one each.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhvnnnD5fTFupK8WWq35un

crates/engine/src/cmd/plugin.rs

@@ -59,6 +59,10 @@ fn plugin_by_id(cmd: &str, p: &Value) -> Result<Arc<Plugin>> {
     registry::get(id).ok_or_else(|| err(vectorcraft_plugins::Error::NotFound(id.into())))
 }
 
+/// `plugin.run`: runs filter `id` on `ids` (else the selected paths and compound paths) with its
+/// parameters and `_context`, and applies its output as one undo step ([`objects::apply_output`];
+/// effect records as [`admit_effects`] admits them). → `{plugin, ids}`: what it made or changed,
+/// which is then selected.
 fn run(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "plugin.run";
     let plugin = plugin_by_id(C, p)?;