feat(ucs): interactive UCS command — UCS Z 90 typable everywhere (F2, #169)

Bare `UCS` now enters an interactive front-end (UcsCommand) that gathers the
option then its value as separate steps, instead of the command line
submitting `UCS` alone on the first Space. The front-end delegates execution
back to the existing inline `UCS <option> …` handler via a new
`CmdResult::Dispatch(String)`, so the coordinate-system math and the
save/persist path are untouched.

Now `UCS Z 90` works the same from the command line (Space-separated),
headless automation, and any source that feeds the active command — closing
the input half of #169. Inline `UCS Z 90` (one-shot / paste) still hits the
inline handler directly.

Second phase of routing input sources through the unified step driver: the
interactive command is driven by feed_command like any tool. Adds headless
test ucs_interactive_inline_args.

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

src/command/mod.rs

@@ -135,6 +135,12 @@ pub enum CmdResult {
     /// End the selection-gather phase and re-dispatch the named command
     /// with the gathered handles installed as the active scene selection.
     Relaunch(String, Vec<Handle>),
+    /// End the command and dispatch the given command string. Used by an
+    /// interactive front-end that gathered its arguments step-by-step and
+    /// delegates execution to an existing inline command handler (e.g. UCS
+    /// collecting an option + value, then running `UCS Z 90`). Unlike
+    /// `Relaunch` it does not touch the selection.
+    Dispatch(String),
     /// Move `dest` entities to the layer of the `src` entity; end command.
     MatchEntityLayer { dest: Vec<Handle>, src: Handle },
     /// Copy all visual properties (layer/color/linetype/lineweight) from `src` to `dest`; end command.