Implement the 013-hp-status-indicators feature that adds visual HP status indicators to combatant rows with a pure domain function deriving bloodied/unconscious states

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/013-hp-status-indicators/data-model.md

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+# Data Model: HP Status Indicators
+
+**Feature**: 013-hp-status-indicators
+**Date**: 2026-03-05
+
+## Entities
+
+### HpStatus (derived type -- not stored)
+
+A discriminated string union representing the health state of a combatant.
+
+| Value | Condition | Description |
+|-------|-----------|-------------|
+| `"healthy"` | `currentHp >= maxHp / 2` | Combatant is above half HP |
+| `"bloodied"` | `0 < currentHp < maxHp / 2` | Combatant is below half HP but still conscious |
+| `"unconscious"` | `currentHp <= 0` | Combatant is at or below zero HP |
+
+**Derivation rules**:
+- Only meaningful when both `currentHp` and `maxHp` are defined
+- Returns `undefined` when either value is missing (HP tracking not enabled)
+- Evaluation order: check `unconscious` first (currentHp <= 0), then `bloodied` (currentHp < maxHp / 2), else `healthy`
+
+### Combatant (existing -- unchanged)
+
+No modifications to the `Combatant` interface. HP status is computed on demand from existing fields:
+
+| Field | Type | Used for status |
+|-------|------|----------------|
+| `currentHp` | `number \| undefined` | Compared against thresholds |
+| `maxHp` | `number \| undefined` | Defines the "half HP" bloodied threshold |
+
+## State Transitions
+
+HP status changes are a side effect of existing HP operations (adjustHp, setHp). No new state transitions are introduced.
+
+```text
+healthy ──[take damage below half]──> bloodied
+bloodied ──[take damage to 0]──────> unconscious
+unconscious ──[heal above 0]───────> bloodied or healthy
+bloodied ──[heal above half]───────> healthy
+healthy ──[take massive damage]────> unconscious (skip bloodied)
+```
+
+## Storage Impact
+
+None. HP status is derived at render time from existing persisted `currentHp` and `maxHp` values. No localStorage schema changes.