Expose parallel behavior on input-only domains #341

Normalize cumulative parallel behavior on the existing parallel input domain. Today this bridge is proved inside bounded typed parallel composition.

Extract the possible-history admission argument into the cumulative owner, preserving every transcript mass, and make the bounded implementation reuse it. Admission agreement is required only at histories of nonzero mass.

Acceptance: the same cumulative parallel behavior carries the input-domain and reply-port certificates needed for attachment; bounded parallel keeps its current behavior and API.

Prerequisite for #319. Depends on #316.