feat: persist workflow controls and settle stopped branches

internal/storage/mysql/migrations/000021_workflow_stop_intent.up.sql

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+-- 独立保存首次停止意图，避免进入核查后丢失用户取消/失败结清的目标；不增加外键。
+ALTER TABLE weave_instances
+ ADD COLUMN stop_kind VARCHAR(16) CHARACTER SET ascii COLLATE ascii_bin NULL COMMENT '首次停止原因：CANCEL为用户取消，FAILURE为固定失败策略；为空表示尚未要求结清',
+ ADD COLUMN stop_requested_at DATETIME(6) NULL COMMENT '首次停止意图的数据库时间，重报、进入核查或后来取消均不覆盖';
+
+-- 历史取消保留原意图时间；旧失败结清及失败策略触发的核查保留原更新时间作为兼容边界。
+UPDATE weave_instances i SET stop_kind=CASE WHEN cancel_requested_at IS NOT NULL THEN 'CANCEL' ELSE 'FAILURE' END,
+ stop_requested_at=COALESCE(cancel_requested_at,updated_at)
+ WHERE state NOT IN ('SUCCEEDED','FAILED','CANCELLED') AND
+ (cancel_requested_at IS NOT NULL OR state='CANCELLING' OR
+ (JSON_UNQUOTE(JSON_EXTRACT(policy_snapshot,'$.failure_policy'))='FAIL_AFTER_SETTLED' AND EXISTS
+ (SELECT 1 FROM weave_steps s WHERE s.namespace_id=i.namespace_id AND s.workflow_id=i.id AND s.`key`<>'$root' AND s.state IN ('FAILED','CANCELLED','NEEDS_REVIEW'))));
+
+ALTER TABLE weave_instances
+ ADD CONSTRAINT ck_workflow_stop_intent CHECK ((stop_kind IS NULL AND stop_requested_at IS NULL) OR (stop_kind IS NOT NULL AND stop_kind IN ('CANCEL','FAILURE') AND stop_requested_at IS NOT NULL));
+
+-- 结清复用根检查点，已有根只提前检查，不更改固定版本、原预算和截止。
+INSERT INTO weave_pending_advances(namespace_id,id,workflow_id,step_id,expected_step_revision,checkpoint,available_at)
+ SELECT i.namespace_id,RANDOM_BYTES(16),i.id,s.id,s.revision,JSON_OBJECT('schema_version',1,'kind','ROOT'),UTC_TIMESTAMP(6)
+ FROM weave_instances i JOIN weave_steps s ON s.namespace_id=i.namespace_id AND s.workflow_id=i.id AND s.`key`='$root'
+ JOIN weave_activations a ON a.namespace_id=s.namespace_id AND a.workflow_id=s.workflow_id AND a.step_id=s.id AND a.id=s.current_activation_id
+ WHERE i.stop_kind IS NOT NULL AND i.state NOT IN ('SUCCEEDED','FAILED','CANCELLED') AND s.state='ACTIVE' AND a.state='ACTIVE'
+ ON DUPLICATE KEY UPDATE available_at=LEAST(weave_pending_advances.available_at,VALUES(available_at));
+
+UPDATE pulse_execution_gates g JOIN weave_instances i ON i.namespace_id=g.namespace_id AND i.gate_id=g.id
+ SET g.paused=TRUE,g.revision=g.revision+1,g.updated_at=UTC_TIMESTAMP(6)
+ WHERE i.stop_kind IS NOT NULL AND i.state NOT IN ('SUCCEEDED','FAILED','CANCELLED') AND g.paused=FALSE;
+
+-- 反向核对流程所有者，防止无外键时遗漏门控被误改的历史任务。
+ALTER TABLE pulse_tasks ADD KEY ix_task_workflow_owner (namespace_id,owner_kind,owner_id,gate_id,id);