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>
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