docs/emitter-design: ParamShape varies by target, which §2.2 rules out of the plan #698

Problem

§2.2 says plan policy "is document-level, not per-target — the plan is language-neutral by definition, so there is nothing per-language to vary in it". §3.1 puts ParamShape (positional or options-bag) in OpPlan, decided by ParamShapeThreshold in plan.Policy. What reads naturally differs by language: a language with named or keyword arguments tolerates many parameters; one without reaches for an options struct much sooner. A single document-level threshold forces one choice on every target.

Proposal

Move the positional-or-bag decision to refine, as per-target policy, and keep only neutral facts in the plan (counts of path, query, header and body inputs, which inputs are required).