# Combat UI: Unit Info Panel

## Overview

Replace the current exit-button-only CombatUI with a unit info panel that displays the selected unit's name and HP. The panel appears in the bottom-left corner when a unit is selected and hides when deselected.

## Scene Structure

```
CombatUI (CanvasLayer) — script: scripts/combat_ui.gd
  └─ UnitPanel (PanelContainer) — anchored bottom-left, hidden by default
       └─ MarginContainer
            └─ VBoxContainer
                 ├─ NameLabel (Label) — displays unit name
                 └─ HPBar (ProgressBar) — displays current_hp / max_hp
```

- The existing exit button and its VBoxContainer are removed entirely.
- The CombatUI CanvasLayer is preserved; its theme reference to `main_ui_theme.tres` stays.

## Layout

- UnitPanel is anchored to the bottom-left of the viewport.
- Margin of 8px from the left and bottom edges.
- Panel width: ~200px (enough for name and HP bar).
- Height: determined by contents.

## Behavior

### Unit Discovery

CombatUI finds units by looking for nodes in the `"units"` group. Units must be added to this group — this is done in `unit.tscn` (scene-level group assignment) or in `unit.gd`'s `_ready()`.

### Signal Connection

On `_ready()`, CombatUI:
1. Calls `get_tree().get_nodes_in_group("units")` to find all current units.
2. Connects each unit's `unit_selected_changed(unit: Unit, selected: bool)` signal to a handler.

### Selection Handling

When `unit_selected_changed` is received:
- If `selected == true`: populate NameLabel with `unit.current_info.name`, set HPBar's `max_value` to `unit.current_stats.max_hp` and `value` to `unit.current_stats.current_hp`, show UnitPanel.
- If `selected == false`: hide UnitPanel.

### Edge Cases

- If no units exist in the group at ready time, the panel simply stays hidden.
- If a unit is removed from the scene (e.g., defeated), the signal disconnects naturally with the node. The panel hides if that unit was selected.

## Files Changed

| File | Change |
|------|--------|
| `prefabs/combat_ui.tscn` | Remove exit button/VBoxContainer. Add UnitPanel with NameLabel and HPBar. Attach new script. |
| `scripts/combat_ui.gd` | New script: connects to unit signals, shows/hides panel, populates data. |
| `prefabs/unit.tscn` | Add the node to the `"units"` group. |

## Styling

Uses the existing `main_ui_theme.tres` applied at the CombatUI or UnitPanel level. No new theme resources needed.

## Out of Scope

- Full stat display (ATK, DEF, etc.) — future expansion.
- Animations (fade/slide) — instant show/hide only.
- Pause menu / exit button — separate future feature.
- HP bar color theming — uses default ProgressBar style from theme.