Block dispatch and force the execution down

Stops the session by force, in any admission state but closed: dispatch is blocked, queued input is cancelled (each input's receipt fails with SESSION_STOPPED), open requests close and the execution is killed. The session goes stopping until the kill is observed, then stopped; with nothing to kill it is stopped at once and the receipt is succeeded in the response. A recovery_required session keeps that state. A turn the kill cut short may end outcome_unknown, which holds the session in recovery_required until POST /v1/sessions/{id}/recovery-decisions. The kill reverts nothing the execution already did outside (commits, pushes, API calls): the response and the receipt say external_effects_reverted: false. The receipt result is {execution_gone, checkpoint_revision, unconfirmed_turn_id, external_effects_reverted}. Resume a stopped session from its last checkpoint with POST /v1/sessions/{id}/resume. A pause or close still draining is overtaken: its receipt fails with CONTROL_SUPERSEDED. To end a session whose work is done, close it instead (POST /v1/sessions/{id}/close).

Scope: sessions:control (API key or cookie session). Idempotency-Key: required.