feat: module 06 parser

First Part II module. The cumulative reference adds a second engine
(src/engine/puzzle: grid, lexer, parser) beside Part I's; nouns and
properties are data in src/content/words.json, and the engine names only
IS/AND/NOT/TEXT and the base rule TEXT IS PUSH. Rules are recompiled from
the board on every change; rules() is a sorted set. Adds the M6 scenarios
and tests (rows, columns, shared IS, AND distribution, stacked text, base
rule), the HOT word as a data-only Expansion Drop, the lesson with the
Part II bridge, and the Rust LevelDef shape in harness.md.

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

course/06-parser/tests/rust/test_06.rs

@@ -0,0 +1,118 @@
+// Module 6: Grid, lexer and parser. Each test matches a heading in scenarios.md.
+use engine::harness::*;
+
+/// An empty level `width` wide and `height` high.
+fn empty_level(width: i32, height: i32) -> LevelDef {
+    LevelDef {
+        width,
+        height,
+        ..Default::default()
+    }
+}
+
+fn object(kind: &str, x: i32, y: i32) -> LevelObject {
+    LevelObject {
+        kind: kind.to_string(),
+        x,
+        y,
+    }
+}
+
+/// Place one text object per word: `(word, x, y)`.
+fn write(b: &mut Baba, words: &[(&str, i32, i32)]) {
+    for &(word, x, y) in words {
+        b.place(&format!("text:{word}"), x, y);
+    }
+}
+
+#[test]
+fn test_06_horizontal_rule() {
+    let b = new_baba(LevelDef {
+        width: 5,
+        height: 3,
+        objects: vec![
+            object("text:BABA", 0, 0),
+            object("text:IS", 1, 0),
+            object("text:YOU", 2, 0),
+            object("baba", 3, 2),
+            object("text:YOU", 0, 2),
+            object("text:IS", 1, 2),
+            object("text:FLAG", 2, 2),
+        ],
+        ..Default::default()
+    });
+
+    assert_eq!(b.rules(), vec!["BABA IS YOU", "TEXT IS PUSH"]);
+}
+
+#[test]
+fn test_06_crossword_shared_is() {
+    let mut b = new_baba(empty_level(3, 3));
+    write(
+        &mut b,
+        &[("BABA", 0, 1), ("IS", 1, 1), ("YOU", 2, 1), ("FLAG", 1, 0)],
+    );
+
+    assert_eq!(b.rules(), vec!["BABA IS YOU", "TEXT IS PUSH"]);
+
+    b.place("text:WIN", 1, 2);
+
+    assert_eq!(
+        b.rules(),
+        vec!["BABA IS YOU", "FLAG IS WIN", "TEXT IS PUSH"]
+    );
+}
+
+#[test]
+fn test_06_and_distributes() {
+    let mut b = new_baba(empty_level(7, 1));
+    write(
+        &mut b,
+        &[
+            ("BABA", 0, 0),
+            ("AND", 1, 0),
+            ("ROCK", 2, 0),
+            ("IS", 3, 0),
+            ("PUSH", 4, 0),
+            ("AND", 5, 0),
+            ("STOP", 6, 0),
+        ],
+    );
+
+    assert_eq!(
+        b.rules(),
+        vec![
+            "BABA IS PUSH",
+            "BABA IS STOP",
+            "ROCK IS PUSH",
+            "ROCK IS STOP",
+            "TEXT IS PUSH"
+        ]
+    );
+}
+
+#[test]
+fn test_06_stacked_text_all_combinations() {
+    let mut b = new_baba(empty_level(3, 1));
+    write(
+        &mut b,
+        &[("BABA", 0, 0), ("ROCK", 0, 0), ("IS", 1, 0), ("YOU", 2, 0)],
+    );
+
+    assert_eq!(
+        b.rules(),
+        vec!["BABA IS YOU", "ROCK IS YOU", "TEXT IS PUSH"]
+    );
+}
+
+#[test]
+fn test_06_default_text_is_push() {
+    let b = new_baba(LevelDef {
+        width: 3,
+        height: 3,
+        objects: vec![object("baba", 1, 1)],
+        ..Default::default()
+    });
+
+    assert_eq!(b.rules(), vec!["TEXT IS PUSH"]);
+}