Add level 6: Anchors

Target structure: a `trap:` dict plus a `rooms:` map where 2-3 rooms
all share the same trap. serde_yaml expands aliases on parse, so the
emitted target is the inlined form; players using `&anchor` / `*alias`
produce the same parsed Value and pass via the semantic short-circuit.

Trap properties (type / depth / spikes) and reused rooms are randomised
per seed (ChaCha8Rng XOR'd with 0x..06).

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/l06_anchors.rs

@@ -0,0 +1,113 @@
+//! Level 6 — anchors. Two rooms share the same trap — define it once.
+//!
+//! Paired design note: `l06.md`.
+//!
+//! Note: serde_yaml resolves aliases at parse time, so the target is
+//! emitted **expanded** (the trap dict appears in each room). Players
+//! who use anchors/aliases will produce the same parsed `Value` and
+//! pass via the semantic short-circuit. Players who paste the dict
+//! verbatim also pass.
+
+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 Anchors;
+
+const ROOM_NAMES: &[&str] = &["north", "south", "east", "west"];
+const TRAP_TYPES: &[&str] = &["pit", "snare", "dart", "rune"];
+
+#[derive(Serialize)]
+struct DescCtx {
+    trap_type: String,
+    trap_depth: i64,
+    trap_spikes: bool,
+    rooms: Vec<String>,
+}
+
+impl Level for Anchors {
+    fn id(&self) -> u8 {
+        6
+    }
+
+    fn name(&self) -> &'static str {
+        "Anchors"
+    }
+
+    fn generate(&self, seed: u64) -> Generated {
+        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_0006);
+        let trap_type = *TRAP_TYPES.choose(&mut rng).expect("non-empty");
+        let trap_depth = rng.gen_range(10..=30i64);
+        let trap_spikes = rng.gen_bool(0.5);
+        let n_rooms = rng.gen_range(2..=3);
+        let rooms: Vec<&'static str> = ROOM_NAMES
+            .choose_multiple(&mut rng, n_rooms)
+            .copied()
+            .collect();
+
+        let mut trap = Mapping::new();
+        trap.insert(
+            Value::String("type".to_string()),
+            Value::String(trap_type.to_string()),
+        );
+        trap.insert(Value::String("depth".to_string()), Value::from(trap_depth));
+        trap.insert(
+            Value::String("spikes".to_string()),
+            Value::Bool(trap_spikes),
+        );
+
+        let mut rooms_map = Mapping::new();
+        for r in &rooms {
+            rooms_map.insert(
+                Value::String((*r).to_string()),
+                Value::Mapping(trap.clone()),
+            );
+        }
+
+        let mut top = Mapping::new();
+        top.insert(Value::String("trap".to_string()), Value::Mapping(trap));
+        top.insert(
+            Value::String("rooms".to_string()),
+            Value::Mapping(rooms_map),
+        );
+
+        let target_yaml =
+            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");
+
+        let mut d = Describer::new();
+        d.register(
+            "l06",
+            "A single trap recurs through these halls:\n\
+             - type: {{ trap_type }}\n\
+             - depth: {{ trap_depth }}\n\
+             - spikes: {{ trap_spikes }}\n\
+             \n\
+             Reuse it for these rooms: {% for r in rooms %}{{ r }}{% if not loop.last %}, {% endif %}{% endfor %}.\n\
+             💡 Define `trap: &name` once and reference it as `*name` in every room.",
+        )
+        .expect("register template");
+        let description = d
+            .render(
+                "l06",
+                &DescCtx {
+                    trap_type: trap_type.to_string(),
+                    trap_depth,
+                    trap_spikes,
+                    rooms: rooms.iter().map(|s| s.to_string()).collect(),
+                },
+            )
+            .expect("render template");
+
+        Generated {
+            target_yaml,
+            description,
+            flavor: "🪤 A trap recurs through these halls.".to_string(),
+        }
+    }
+}

src/levels/l09_operators.rs

@@ -1,135 +0,0 @@
-//! 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

@@ -13,7 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
-pub mod l09_operators;
+pub mod l06_anchors;
 
 use serde::{Deserialize, Serialize};