Add level 4: The Chest (lists)

Top-level `chest:` key with a 3-5 item sequence drawn from a small pool.
Per-seed deterministic via ChaCha8Rng XOR'd with 0x..04. Description
rendered with tera; flavor uses a 📦 emoji.

Not wired into levels::registry() yet — integration belongs to a follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/levels/l04_list.rs

@@ -0,0 +1,71 @@
+//! Level 4 — lists. A chest of loot lies open before you.
+//!
+//! Paired design note: `l04.md`.
+
+use rand::seq::SliceRandom;
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
+use serde::Serialize;
+use serde_yaml::{Mapping, Sequence, Value};
+
+use crate::describe::Describer;
+
+use super::{Generated, Level};
+
+pub struct List;
+
+const ITEMS: &[&str] = &[
+    "sword", "torch", "rope", "bread", "dagger", "scroll", "gem", "coin", "potion", "shield",
+];
+
+#[derive(Serialize)]
+struct DescCtx {
+    items: Vec<String>,
+}
+
+impl Level for List {
+    fn id(&self) -> u8 {
+        4
+    }
+
+    fn name(&self) -> &'static str {
+        "The Chest"
+    }
+
+    fn generate(&self, seed: u64) -> Generated {
+        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_0004);
+        let n = rng.gen_range(3..=5);
+        let items: Vec<&'static str> = ITEMS.choose_multiple(&mut rng, n).copied().collect();
+
+        let seq: Sequence = items
+            .iter()
+            .map(|i| Value::String((*i).to_string()))
+            .collect();
+
+        let mut top = Mapping::new();
+        top.insert(Value::String("chest".to_string()), Value::Sequence(seq));
+        let target_yaml =
+            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");
+
+        let mut d = Describer::new();
+        d.register(
+            "l04",
+            "A chest lies open. Inside:\n{% for it in items %}- {{ it }}\n{% endfor %}\n💡 Wrap the items as a YAML list under a `chest:` key.",
+        )
+        .expect("register template");
+        let description = d
+            .render(
+                "l04",
+                &DescCtx {
+                    items: items.iter().map(|s| s.to_string()).collect(),
+                },
+            )
+            .expect("render template");
+
+        Generated {
+            target_yaml,
+            description,
+            flavor: "📦 A chest of loot lies open before you.".to_string(),
+        }
+    }
+}

src/levels/mod.rs

@@ -13,6 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
+pub mod l04_list;
 
 use serde::{Deserialize, Serialize};