Add level 9: Merge Keys (special operators)

`door_defaults` plus `north_door` and `south_door`, each containing a
`<<:` key whose value is the defaults dict. north overrides locked,
south overrides material. serde_yaml treats `<<` as a literal mapping
key, so the target and a player's `<<: *defaults` form parse to the
same Value.

Default material/locked and which side overrides which are randomised
per seed (ChaCha8Rng XOR'd with 0x..09).

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/l09_operators.rs

@@ -0,0 +1,135 @@
+//! Level 9 — special operators. The merge key (`<<`) lets a door
+//! inherit a template and override one field.
+//!
+//! Paired design note: `l09.md`.
+//!
+//! `serde_yaml` treats `<<` as a literal mapping key (it does NOT
+//! perform YAML 1.1 merge-key resolution). The target therefore carries
+//! a `<<` key whose value is the defaults dict, exactly as the player's
+//! `<<: *defaults` parses.
+
+use rand::seq::SliceRandom;
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
+use serde::Serialize;
+use serde_yaml::{Mapping, Value};
+
+use crate::describe::Describer;
+
+use super::{Generated, Level};
+
+pub struct Operators;
+
+const MATERIALS: &[&str] = &["oak", "iron", "stone", "silver", "bone"];
+
+#[derive(Serialize)]
+struct DescCtx {
+    default_material: String,
+    default_locked: bool,
+    north_locked: bool,
+    south_material: String,
+}
+
+impl Level for Operators {
+    fn id(&self) -> u8 {
+        9
+    }
+
+    fn name(&self) -> &'static str {
+        "Merge Keys"
+    }
+
+    fn generate(&self, seed: u64) -> Generated {
+        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_0009);
+        let default_material = (*MATERIALS.choose(&mut rng).unwrap()).to_string();
+        let default_locked = rng.gen_bool(0.5);
+        // The north door overrides locked; choose the opposite so the
+        // override is meaningful.
+        let north_locked = !default_locked;
+        // The south door overrides material; pick anything but default.
+        let other_materials: Vec<&str> = MATERIALS
+            .iter()
+            .filter(|m| **m != default_material)
+            .copied()
+            .collect();
+        let south_material = (*other_materials.choose(&mut rng).unwrap()).to_string();
+
+        // Build the defaults mapping (referenced by all doors).
+        let mut defaults = Mapping::new();
+        defaults.insert(
+            Value::String("material".to_string()),
+            Value::String(default_material.clone()),
+        );
+        defaults.insert(
+            Value::String("locked".to_string()),
+            Value::Bool(default_locked),
+        );
+
+        let mut north_door = Mapping::new();
+        north_door.insert(
+            Value::String("<<".to_string()),
+            Value::Mapping(defaults.clone()),
+        );
+        north_door.insert(
+            Value::String("locked".to_string()),
+            Value::Bool(north_locked),
+        );
+
+        let mut south_door = Mapping::new();
+        south_door.insert(
+            Value::String("<<".to_string()),
+            Value::Mapping(defaults.clone()),
+        );
+        south_door.insert(
+            Value::String("material".to_string()),
+            Value::String(south_material.clone()),
+        );
+
+        let mut top = Mapping::new();
+        top.insert(
+            Value::String("door_defaults".to_string()),
+            Value::Mapping(defaults),
+        );
+        top.insert(
+            Value::String("north_door".to_string()),
+            Value::Mapping(north_door),
+        );
+        top.insert(
+            Value::String("south_door".to_string()),
+            Value::Mapping(south_door),
+        );
+
+        let target_yaml =
+            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");
+
+        let mut d = Describer::new();
+        d.register(
+            "l09",
+            "Two doors share a template:\n\
+             default material: {{ default_material }}\n\
+             default locked:   {{ default_locked }}\n\
+             \n\
+             north_door overrides locked → {{ north_locked }}\n\
+             south_door overrides material → {{ south_material }}\n\
+             💡 Anchor the defaults (`door_defaults: &name`) and merge with `<<: *name` in each door.",
+        )
+        .expect("register template");
+        let description = d
+            .render(
+                "l09",
+                &DescCtx {
+                    default_material,
+                    default_locked,
+                    north_locked,
+                    south_material,
+                },
+            )
+            .expect("render template");
+
+        Generated {
+            target_yaml,
+            description,
+            flavor: "🚪 Two doors echo a single template.".to_string(),
+        }
+    }
+}

src/levels/mod.rs

@@ -13,6 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
+pub mod l09_operators;
 
 use serde::{Deserialize, Serialize};