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/l10_dynamic.rs

@@ -1,101 +0,0 @@
-//! Level 10 — dynamic values. The vault ledger accepts numbers and
-//! dates in many forms.
-//!
-//! Paired design note: `l10.md`.
-//!
-//! The target is canonical (decimal ints, decimal float, ISO date
-//! string). The lesson is that the player can write equivalent values
-//! in hex / octal / exponent forms — all parse to the same number.
-
-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 Dynamic;
-
-#[derive(Serialize)]
-struct DescCtx {
-    gold_dec: i64,
-    gold_hex: String,
-    silver_dec: i64,
-    silver_oct: String,
-    experience: f64,
-    date: String,
-}
-
-impl Level for Dynamic {
-    fn id(&self) -> u8 {
-        10
-    }
-
-    fn name(&self) -> &'static str {
-        "Vault Ledger"
-    }
-
-    fn generate(&self, seed: u64) -> Generated {
-        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_000A);
-
-        let gold = rng.gen_range(0x100..=0xFFFi64);
-        let silver = rng.gen_range(0o100..=0o777i64);
-        // Fractional float so the canonical serialisation keeps the `.5`.
-        let experience = rng.gen_range(10..=99) as f64 + 0.5;
-        let year = rng.gen_range(1100..=2050i64);
-        let month = rng.gen_range(1..=12i64);
-        let day = rng.gen_range(1..=28i64);
-        let date = format!("{year:04}-{month:02}-{day:02}");
-
-        let mut vault = Mapping::new();
-        vault.insert(Value::String("gold".to_string()), Value::from(gold));
-        vault.insert(Value::String("silver".to_string()), Value::from(silver));
-        vault.insert(
-            Value::String("experience".to_string()),
-            Value::from(experience),
-        );
-        vault.insert(
-            Value::String("date".to_string()),
-            Value::String(date.clone()),
-        );
-
-        let mut top = Mapping::new();
-        top.insert(Value::String("vault".to_string()), Value::Mapping(vault));
-
-        let target_yaml =
-            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");
-
-        let mut d = Describer::new();
-        d.register(
-            "l10",
-            "The vault ledger demands its values:\n\
-             gold:       {{ gold_dec }}   (try hex: 0x{{ gold_hex }})\n\
-             silver:     {{ silver_dec }} (try octal: 0o{{ silver_oct }})\n\
-             experience: {{ experience }} (any equivalent float form passes)\n\
-             date:       {{ date }}        (an ISO-8601 date string)\n\
-             💡 Any equivalent numeric form passes — what matters is the parsed value.",
-        )
-        .expect("register template");
-        let description = d
-            .render(
-                "l10",
-                &DescCtx {
-                    gold_dec: gold,
-                    gold_hex: format!("{:X}", gold),
-                    silver_dec: silver,
-                    silver_oct: format!("{:o}", silver),
-                    experience,
-                    date,
-                },
-            )
-            .expect("render template");
-
-        Generated {
-            target_yaml,
-            description,
-            flavor: "🪙 A vault ledger awaits in many ciphers.".to_string(),
-        }
-    }
-}

src/levels/mod.rs

@@ -13,7 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
-pub mod l10_dynamic;
+pub mod l09_operators;
 
 use serde::{Deserialize, Serialize};