fix: add Knapsack re-export and edge-case tests

- Re-export Knapsack from models/mod.rs (structural review finding)
- Add tests: zero capacity, single item, greedy-not-optimal adversarial case

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zazabap

src/models/mod.rs

@@ -15,5 +15,5 @@ pub use graph::{
     BicliqueCover, KColoring, MaxCut, MaximalIS, MaximumClique, MaximumIndependentSet,
     MaximumMatching, MinimumDominatingSet, MinimumVertexCover, SpinGlass, TravelingSalesman,
 };
-pub use misc::{BinPacking, Factoring, PaintShop};
+pub use misc::{BinPacking, Factoring, Knapsack, PaintShop};
 pub use set::{MaximumSetPacking, MinimumSetCovering};

src/unit_tests/models/misc/knapsack.rs

@@ -86,6 +86,41 @@ fn test_knapsack_serialization() {
     assert_eq!(restored.capacity(), problem.capacity());
 }
 
+#[test]
+fn test_knapsack_zero_capacity() {
+    // Capacity 0: only empty set is feasible
+    let problem = Knapsack::new(vec![1, 2], vec![10, 20], 0);
+    assert_eq!(problem.evaluate(&[0, 0]), SolutionSize::Valid(0));
+    assert_eq!(problem.evaluate(&[1, 0]), SolutionSize::Invalid);
+    let solver = BruteForce::new();
+    let solution = solver.find_best(&problem).unwrap();
+    assert_eq!(problem.evaluate(&solution), SolutionSize::Valid(0));
+}
+
+#[test]
+fn test_knapsack_single_item() {
+    // Single item that fits
+    let problem = Knapsack::new(vec![3], vec![5], 3);
+    assert_eq!(problem.evaluate(&[1]), SolutionSize::Valid(5));
+    assert_eq!(problem.evaluate(&[0]), SolutionSize::Valid(0));
+    let solver = BruteForce::new();
+    let solution = solver.find_best(&problem).unwrap();
+    assert_eq!(problem.evaluate(&solution), SolutionSize::Valid(5));
+}
+
+#[test]
+fn test_knapsack_greedy_not_optimal() {
+    // Classic case where greedy by value/weight ratio is suboptimal:
+    // Item 0: w=6, v=7, ratio=1.17 (greedy picks this first, then nothing else fits)
+    // Item 1: w=5, v=5, ratio=1.00
+    // Item 2: w=5, v=5, ratio=1.00
+    // Capacity=10. Greedy: {0} value=7. Optimal: {1,2} value=10.
+    let problem = Knapsack::new(vec![6, 5, 5], vec![7, 5, 5], 10);
+    let solver = BruteForce::new();
+    let solution = solver.find_best(&problem).unwrap();
+    assert_eq!(problem.evaluate(&solution), SolutionSize::Valid(10));
+}
+
 #[test]
 #[should_panic(expected = "weights and values must have the same length")]
 fn test_knapsack_mismatched_lengths() {