Single-token KEY=value split for direct lower_command callers and
exotic keys the grammar cannot classify (single tokens only — no whitespace
reassembly, so the quoted-space corruption class cannot arise here).
Returns Ok(None) when there is no =.
Validate positional args against a command’s declared specs.
Missing required positionals, statically-known type violations, and
trailing positionals beyond a fixed-arity spec all fail here;
templates, variables, and fragments defer to the runtime resolvers.
A trailing Rest spec absorbs any number of positionals.