Merge 0.30 release history into 0.31

Reconcile the released 0.30 ancestry after its fixes were incorporated during 0.31 development. Retain the 0.31 policy tests, v9 evidence identities, and multi-Script cost gate. The resulting tree is identical to 648bb83; no compiler, fixture, or submodule content changes.

Validation: identical Git tree to the previously validated 0.31 tip; fresh cellscript_gate.sh dev passed.

Author: a19q3