feat: log tree from Module 2 (logTree / log_tree); SpellGrid CLIs print Log:

Engine events carry the nesting depth they were recorded at; the harness
prints the log as an indented tree. New tests test_02_log_tree_shows_nesting
and test_03_log_tree_death_phase; both SpellGrid CLIs print the tree of the
last command's Sequences under a Log: line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

course/06-parser/reference/rust/src/engine/event.rs

@@ -1,4 +1,5 @@
 //! What happened, in order. The engine records an event each time it changes the game.
+//! The world also keeps the nesting depth each event was recorded at (`World::log_depths`).
 
 use super::card::TriggerOn;
 use super::world::{EntityId, PlayerId};

course/06-parser/reference/rust/src/engine/queue.rs

@@ -246,8 +246,11 @@ fn react(world: &mut World, events: &[Event]) {
                 }
                 _ => world.board_index(source),
             };
+            // The trigger's consequences are one level deeper in the log tree.
+            world.open_block();
             let owner = world.owner_of(source);
             run_effects(world, &trigger.effects, owner, Some(source), None, position);
+            world.close_block();
         }
     }
 }

course/06-parser/reference/rust/src/engine/world.rs

@@ -84,6 +84,10 @@ pub struct World {
     sides: [Side; 2],
     next_order_of_play: u32,
     log: Vec<Event>,
+    /// The nesting depth at which each event in `log` was recorded, index for index.
+    depths: Vec<u32>,
+    /// How many triggers are resolving right now, one inside another.
+    depth: u32,
     /// The auras in force as of the last Aura Update. Changes only there (see `aura.rs`).
     auras: Vec<ActiveAura>,
 }
@@ -99,6 +103,8 @@ impl World {
             sides: Default::default(),
             next_order_of_play: 0,
             log: Vec::new(),
+            depths: Vec::new(),
+            depth: 0,
             auras: Vec::new(),
         };
         for owner in 0..2 {
@@ -287,10 +293,27 @@ impl World {
         &self.log
     }
 
+    /// The nesting depth of each event in `log()`, index for index: 0 for the outermost
+    /// action's own events.
+    pub fn log_depths(&self) -> &[u32] {
+        &self.depths
+    }
+
     // ---- Changes: each one records its event ----------------------------
 
     pub fn record(&mut self, event: Event) {
         self.log.push(event);
+        self.depths.push(self.depth);
+    }
+
+    /// A trigger starts resolving: the events it causes are recorded one level deeper.
+    pub fn open_block(&mut self) {
+        self.depth += 1;
+    }
+
+    /// The trigger that last opened a block has finished resolving.
+    pub fn close_block(&mut self) {
+        self.depth -= 1;
     }
 
     /// Add a card to the library, or replace the card with the same name.
@@ -325,6 +348,7 @@ impl World {
 
     pub fn clear_log(&mut self) {
         self.log.clear();
+        self.depths.clear();
     }
 
     pub fn damage(&mut self, target: EntityId, amount: i32) {

course/06-parser/reference/rust/src/harness.rs

@@ -55,6 +55,48 @@ impl GameEvent {
     }
 }
 
+/// One event as `scenarios.md` writes it, such as `Triggered{source: 3, trigger: "deathrattle"}`.
+impl std::fmt::Display for GameEvent {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            GameEvent::Summoned { id } => write!(f, "Summoned{{id: {id}}}"),
+            GameEvent::Damaged { target, amount } => {
+                write!(f, "Damaged{{target: {target}, amount: {amount}}}")
+            }
+            GameEvent::Triggered { source, trigger } => {
+                write!(f, "Triggered{{source: {source}, trigger: {trigger:?}}}")
+            }
+            GameEvent::Destroyed { id } => write!(f, "Destroyed{{id: {id}}}"),
+            GameEvent::DeathPhase { round } => write!(f, "DeathPhase{{round: {round}}}"),
+            GameEvent::Drew { player, id } => write!(f, "Drew{{player: {player}, id: {id}}}"),
+            GameEvent::Fatigue { player } => write!(f, "Fatigue{{player: {player}}}"),
+            GameEvent::ArmorGained { player, amount } => {
+                write!(f, "ArmorGained{{player: {player}, amount: {amount}}}")
+            }
+            GameEvent::StatsChanged { id, attack, health } => {
+                write!(
+                    f,
+                    "StatsChanged{{id: {id}, attack: {attack}, health: {health}}}"
+                )
+            }
+            GameEvent::SpellCast { card } => write!(f, "SpellCast{{card: {card:?}}}"),
+        }
+    }
+}
+
+/// The log as a tree: one event per line, two spaces of indent per level of nesting.
+fn log_tree(world: &World, view: impl Fn(&Event) -> Option<GameEvent>) -> String {
+    world
+        .log()
+        .iter()
+        .zip(world.log_depths())
+        .filter_map(|(event, &depth)| {
+            let shown = view(event)?;
+            Some(format!("{}{shown}\n", "  ".repeat(depth as usize)))
+        })
+        .collect()
+}
+
 pub struct Game {
     world: World,
 }
@@ -146,6 +188,10 @@ impl Game {
     pub fn clear_log(&mut self) {
         self.world.clear_log();
     }
+    /// The log as an indented tree (see course/harness.md).
+    pub fn log_tree(&self) -> String {
+        log_tree(&self.world, GameEvent::from_engine)
+    }
 
     // ---- Module 3 ----