refactor(commands): unified StepInput → feed_command step router (F0)

Introduce the single place every input source funnels through to drive the
active command's step machine: a `StepInput` vocabulary (one variant per
`CadCommand::on_*` method) and `OpenCADStudio::feed_command`, which routes
the input to the matching `on_*` and applies the result. Sources keep their
own classification (raw input → which StepInput); only the routing is
centralized.

Migrate the headless automation feeder onto it: feed_active_cmd builds a
StepInput, the terminator sends StepInput::Enter. No behavior change —
automation_ops_round_trip still passes.

Also fixes headless inline-argument loss for plugin commands (#162):
run_headless now offers the whole line to plugin dispatch first (plugins
parse their own inline args, e.g. `HC_PIPE 2B 2C 1.25 0.013`), falling back
to the first-word + fed-tokens path for built-in interactive tools.

First phase of routing all input sources (command line, dynamic input,
plugin API, viewport clicks) through one driver.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/command/mod.rs

@@ -46,6 +46,39 @@ pub enum TangentObject {
     Circle { center: DVec3, radius: f64 },
 }
 
+/// One unit of input to the active command's step machine.
+///
+/// Every input source — the GUI command line, the headless automation feeder,
+/// dynamic input, the plugin API, and the viewport (clicks / picks / selection
+/// / tangent) — translates its raw input into one of these and routes it
+/// through `OpenCADStudio::feed_command`, so a single place drives the command
+/// regardless of where the step came from.
+///
+/// Variants are wired up incrementally as each source is migrated onto
+/// `feed_command`; `#[allow(dead_code)]` covers those not yet constructed.
+#[allow(dead_code)]
+pub enum StepInput {
+    /// A coordinate (`on_point`).
+    Point(DVec3),
+    /// A typed token: keyword, option letter, distance, or value
+    /// (`on_text_input`).
+    Text(String),
+    /// An object pick (`on_entity_pick`).
+    EntityPick(Handle, DVec3),
+    /// A sub-structure pick — vertex / edge / face (`on_structure_pick`).
+    StructurePick(Handle, DVec3),
+    /// A completed selection set (`on_selection_complete`).
+    SelectionComplete(Vec<Handle>),
+    /// A tangent-snap target (`on_tangent_point`).
+    Tangent(TangentObject, DVec3),
+    /// The text / MText editor closed, with its commit flag (`on_editor_closed`).
+    EditorClosed(bool),
+    /// Advance / finish the current step — Enter, or Space-as-Enter (`on_enter`).
+    Enter,
+    /// Cancel the command (`on_escape`).
+    Escape,
+}
+
 // ── Result token ──────────────────────────────────────────────────────────
 
 /// Returned by every `CadCommand` method to tell main.rs what to do.