feat: propagate intent venue through execution

trex_core/adapters/persistence/sql/017_add_slot_order_venue.sql

@@ -0,0 +1,11 @@
+-- Preserve the admitted intent venue until dispatch/cancel. Existing rows are
+-- explicitly backward compatible with the historical KRX-only execution path.
+ALTER TABLE daily_slot
+    ADD COLUMN IF NOT EXISTS venue TEXT NOT NULL DEFAULT 'KRX';
+
+ALTER TABLE daily_slot
+    DROP CONSTRAINT IF EXISTS daily_slot_venue_check;
+
+ALTER TABLE daily_slot
+    ADD CONSTRAINT daily_slot_venue_check
+    CHECK (venue IN ('KRX', 'NXT', 'SOR'));