Add combat HP tracker

- one running tracker (Combatant rows); no encounters to manage
- Enemies list gets a "Track" column (checkbox + count) and an
  "Add to tracker" button; picking N of an enemy makes "Goblin 1..N",
  numbering continues if some are already on the tracker
- /tracker: per-combatant current/max HP with an HP bar, Damage / Heal
  by amount, remove one, "Clear all". HP is clamped to [0, max];
  0 HP shows a "Down" badge. Stats are snapshotted from the Enemy on add.
- "Tracker" nav item; tracker_picker Stimulus controller; app.js gains a
  data-format="pick" cell renderer

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Entity/Combatant.php

@@ -0,0 +1,140 @@
+<?php
+
+namespace App\Entity;
+
+use App\Repository\CombatantRepository;
+use Doctrine\DBAL\Types\Types;
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * One line on the running combat HP tracker. Stats are copied from the Enemy
+ * when added, so editing the Enemy mid-fight doesn't change the tracker.
+ */
+#[ORM\Entity(repositoryClass: CombatantRepository::class)]
+#[ORM\Table(name: 'combatant')]
+class Combatant
+{
+    #[ORM\Id]
+    #[ORM\GeneratedValue]
+    #[ORM\Column]
+    private ?int $id = null;
+
+    #[ORM\ManyToOne(targetEntity: Enemy::class)]
+    #[ORM\JoinColumn(name: 'enemy_id', onDelete: 'SET NULL', nullable: true)]
+    private ?Enemy $enemy = null;
+
+    #[ORM\Column(length: 120)]
+    private string $label = '';
+
+    #[ORM\Column(type: Types::SMALLINT)]
+    private int $maxHp = 1;
+
+    #[ORM\Column(type: Types::SMALLINT)]
+    private int $currentHp = 1;
+
+    #[ORM\Column(type: Types::SMALLINT)]
+    private int $position = 0;
+
+    #[ORM\Column(type: Types::DATETIME_IMMUTABLE)]
+    private \DateTimeImmutable $addedAt;
+
+    public function __construct()
+    {
+        $this->addedAt = new \DateTimeImmutable();
+    }
+
+    public function isDown(): bool
+    {
+        return $this->currentHp <= 0;
+    }
+
+    /** 0-100, for the HP bar. */
+    public function hpPercent(): int
+    {
+        if ($this->maxHp <= 0) {
+            return 0;
+        }
+
+        return (int) round(max(0, min($this->currentHp, $this->maxHp)) / $this->maxHp * 100);
+    }
+
+    public function applyDamage(int $amount): void
+    {
+        $this->currentHp = max(0, $this->currentHp - abs($amount));
+    }
+
+    public function applyHealing(int $amount): void
+    {
+        $this->currentHp = min($this->maxHp, $this->currentHp + abs($amount));
+    }
+
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+
+    public function getEnemy(): ?Enemy
+    {
+        return $this->enemy;
+    }
+
+    public function setEnemy(?Enemy $enemy): static
+    {
+        $this->enemy = $enemy;
+
+        return $this;
+    }
+
+    public function getLabel(): string
+    {
+        return $this->label;
+    }
+
+    public function setLabel(string $label): static
+    {
+        $this->label = $label;
+
+        return $this;
+    }
+
+    public function getMaxHp(): int
+    {
+        return $this->maxHp;
+    }
+
+    public function setMaxHp(int $maxHp): static
+    {
+        $this->maxHp = $maxHp;
+
+        return $this;
+    }
+
+    public function getCurrentHp(): int
+    {
+        return $this->currentHp;
+    }
+
+    public function setCurrentHp(int $currentHp): static
+    {
+        $this->currentHp = $currentHp;
+
+        return $this;
+    }
+
+    public function getPosition(): int
+    {
+        return $this->position;
+    }
+
+    public function setPosition(int $position): static
+    {
+        $this->position = $position;
+
+        return $this;
+    }
+
+    public function getAddedAt(): \DateTimeImmutable
+    {
+        return $this->addedAt;
+    }
+}