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/modules/view/mod.rs

@@ -15,6 +15,7 @@ mod solid;
 mod tile_horiz;
 mod tile_vert;
 mod tool_palettes;
+pub mod ucs_cmd;
 mod ucs_icon;
 mod view_front;
 mod view_iso;