Add placeholder battle audio

docs/ARCHITECTURE.md

@@ -15,6 +15,7 @@
 - `scripts/core/campaign_state.gd`: Thin campaign save state for gold, inventory, roster, completed scenarios, and campaign flags.
 - `scripts/core/battle_state.gd`: Battle rules, unit state, turns, AI, and victory checks.
 - `scripts/scenes/battle_scene.gd`: Rendering, input handling, and HUD wiring.
+- `audio/bgm/*.wav` and `audio/sfx/*.wav`: Placeholder music loops and interface/action stingers.
 - `data/campaign/campaign.json`: Campaign order and scenario paths.
 - `data/defs/*.json`: Officers, classes, terrain, items, and skills.
 - `data/scenarios/*.json`: Scenario definitions.
@@ -37,7 +38,7 @@
 4. Show scenario briefing, including the optional pre-battle shop, Armory, Roster, and Formation setup.
 5. Player selects a unit.
 6. Briefing completion dispatches battle-begin events, including opening dialogue.
-7. Unit moves, attacks, chooses a tactic or consumable item from the side menu, changes equipment, counters, gains EXP, levels up, promotes at class thresholds, or waits. Tactic buttons show MP cost, kind, range, and power. Physical attacks roll hit chance from unit agility and target terrain avoid; misses give reduced EXP.
+7. Unit moves, attacks, chooses a tactic or consumable item from the side menu, changes equipment, counters, gains EXP, levels up, promotes at class thresholds, or waits. Tactic buttons show MP cost, kind, range, and power. Physical attacks roll hit chance from unit agility and target terrain avoid; misses give reduced EXP. The scene plays placeholder SFX for core action and UI feedback.
 8. Player ends turn.
 9. Enemy AI moves, attacks, and can cast damage or healing tactics through the same combat and skill resolvers.
 10. Battle checks scenario-defined defeat conditions first, including commander loss and turn limits, then victory conditions such as enemy defeat or reaching a marked destination tile.
@@ -57,4 +58,6 @@ Joined officers gate whether `requires_joined` player deployments are loaded at
 
 Inventory and campaign flags are copied from `CampaignState` into `BattleState` when a scenario starts. Pre-battle shop stock comes from the loaded scenario's `shop.items` list plus matching `shop.conditional_items` blocks. Purchases are campaign transactions: they spend saved gold immediately, write the save file, and refresh the already-loaded battle inventory before the player begins the battle. Pre-battle Armory equipment changes use the same equipment rules as battle HUD equipment changes, then immediately save merged roster equipment/stat snapshots and inventory stock. Pre-battle Roster uses scenario `roster.max_units`, `roster.required_officers`, and `roster.required_units` to mark optional player deployments as sortie or reserve; reserve units remain in the candidate list but are excluded from board occupancy, drawing, selection, AI targeting, and living-unit victory checks. Non-controllable player units can act as protected escort targets: enemies can attack them and conditions can reference them, but they are skipped by player selection, Armory, Formation, counterattacks, and campaign roster progression snapshots when `persist_progression` is false. Scenario `ai_target_priority` nudges enemy movement and damage-skill scoring toward important targets such as envoys without overriding defeat bonuses or range checks. Pre-battle Formation uses the loaded scenario's `formation.cells` and only mutates current battle unit positions before battle-begin events fire. Destination victory cells from `unit_reaches_tile` conditions are exposed to the scene for objective overlays and tile info. Movement-triggered `unit_reaches_tile` events receive the unit that just moved, so ambushes fire on entry rather than from units already standing on a marker. Briefing data can append matching `briefing.conditional_lines`, and scenario events can use `when.campaign_flags` to branch dialogue, objective changes, and reinforcements from saved campaign choices. Consumable use can restore HP or MP, and in-battle item use plus equipment swaps mutate only the battle copy until victory; defeats and restarts do not spend saved items or preserve in-battle gear changes. Post-battle choices write campaign `flags` only after the result-panel button is pressed; if the player reloads after rewards are saved but before selecting, the pending scenario id restores the victory choice panel without replaying rewards. The next battle button stays locked until the campaign choice save succeeds. Equipment rewards share the same `item_id -> count` inventory stock, cover weapon, armor, and accessory slots, can be equipped from the side HUD before the selected unit moves or acts, and do not appear in the consumable action menu. Already completed scenarios can be replayed without granting duplicate rewards, choices, or replay inventory consumption; pre-battle Shop, Armory, Roster, and Formation are disabled on completed-scenario replays to avoid side effects.
 
+The battle scene owns presentation audio: briefing and result states use menu BGM, active battles use battle BGM, and log/result hooks trigger placeholder SFX without changing battle rules.
+
 The `New Campaign` button clears `user://campaign_save.json`, resets campaign state to the first scenario, and reloads the opening briefing.

docs/ROADMAP.md

@@ -133,7 +133,7 @@
 - Portrait pipeline.
 - Unit sprites and animations.
 - Battle effects.
-- Music and sound.
+- Music and sound. Basic placeholder BGM and SFX now play for menus, battle flow, unit actions, tactics, items, and result states.
 - Chapter UI and visual novel scenes.
 
 ## Milestone 6: Content Expansion