Add shop sell-back

docs/DATA_MODEL.md

@@ -277,7 +277,7 @@ Weapon `range` also uses `[min, max]`. When class and equipment both provide ran
 
 Consumables use `kind: "consumable"` with effect records. `heal_hp` restores HP and `heal_mp` restores MP to a valid target; `Bean` and `Wine` are the first examples. Item use consumes the acting unit's action and only commits to the save file on victory. Weapons, armor, and accessories can be granted through `rewards.items`; they are shown in inventory, can be equipped through the Equip menu before the unit moves or acts, and are not usable from the battle item menu.
 
-Scenario `shop.items` controls which positive-price items appear in the current pre-battle shop. `shop.conditional_items` can append extra stock when saved campaign flags match. Purchases spend campaign gold, increment the campaign inventory count, save immediately, and then refresh the battle inventory copy if a scenario briefing is already loaded. Later campaign data should add finite stock counts, unlock flags, and sell-back rules.
+Scenario `shop.items` controls which positive-price items appear in the current pre-battle shop. `shop.conditional_items` can append extra stock when saved campaign flags match. Purchases spend campaign gold, increment the campaign inventory count, save immediately, and then refresh the battle inventory copy if a scenario briefing is already loaded. The shop can also sell unequipped inventory stock back for 50% of base `price`, saving immediately and refreshing the loaded battle inventory copy. Later campaign data should add finite stock counts and unlock flags.
 
 The pre-battle Armory operates on the currently loaded player deployments and the copied battle inventory, then saves the changed roster entries and inventory counts back into `CampaignState`. Unlike in-battle equipment swaps, Armory changes persist immediately and survive defeat or restart. Completed-scenario replays keep Shop and Armory disabled so replay setup cannot alter the campaign save.
 
@@ -475,7 +475,7 @@ During a battle, item consumption and equipment swaps affect `BattleState.battle
 
 Physical attacks grant normal attack/counter EXP only on hit after the attack/counter exchange resolves. A missed physical attack or counter grants a small miss EXP value, while defeat bonuses still require the attack to hit and defeat the target.
 
-Before a battle starts, `BattleScene` can buy priced items through `CampaignState.try_buy_item()`. A successful purchase updates campaign gold and inventory in the save file immediately, then calls `BattleState.set_inventory_snapshot()` so the upcoming battle sees the new stock. Pre-battle Armory changes call `CampaignState.try_save_prebattle_loadout()`, which merges the currently deployed roster snapshot into the saved roster and saves the adjusted inventory in the same operation. Pre-battle Roster calls `BattleState.try_set_unit_deployed()` to toggle optional player units while enforcing required officers and sortie limits. Pre-battle Formation calls `BattleState.try_set_prebattle_formation()`, which can move a player unit to an open formation cell or swap two player units inside the formation area. Matching `briefing.conditional_lines` and `shop.conditional_items` are merged while the scenario loads, before the briefing panel opens. `post_battle_dialogue` is rendered by the battle scene after victory and before rewards/choices are shown in the result panel. Post-battle choices call `CampaignState.try_apply_post_battle_choice()`, which saves selected flags and rolls back flags, gold, and inventory if the save write fails.
+Before a battle starts, `BattleScene` can buy priced items through `CampaignState.try_buy_item()` and sell unequipped inventory through `CampaignState.try_sell_item()`. Successful shop transactions update campaign gold and inventory in the save file immediately, then call `BattleState.set_inventory_snapshot()` so the upcoming battle sees the new stock. Pre-battle Armory changes call `CampaignState.try_save_prebattle_loadout()`, which merges the currently deployed roster snapshot into the saved roster and saves the adjusted inventory in the same operation. Pre-battle Roster calls `BattleState.try_set_unit_deployed()` to toggle optional player units while enforcing required officers and sortie limits. Pre-battle Formation calls `BattleState.try_set_prebattle_formation()`, which can move a player unit to an open formation cell or swap two player units inside the formation area. Matching `briefing.conditional_lines` and `shop.conditional_items` are merged while the scenario loads, before the briefing panel opens. `post_battle_dialogue` is rendered by the battle scene after victory and before rewards/choices are shown in the result panel. Post-battle choices call `CampaignState.try_apply_post_battle_choice()`, which saves selected flags and rolls back flags, gold, and inventory if the save write fails.
 
 When every campaign scenario id is present in `completed_scenarios`, the battle scene shows a campaign completion panel. Starting a new campaign clears the save file and resets `current_scenario_id` to `start_scenario`.