v2: add 1024-token long context test

Tests/01_Model/LongContext12BTest.swift

@@ -53,4 +53,37 @@ final class LongContext12BTest: XCTestCase {
             print("  gen[\(i)]: token=\(maxIdx) logit=\(maxVal)")
         }
     }
+
+    func testLongContext1024Tokens() throws {
+        try XCTSkipIf(model == nil, "12B model not found")
+
+        let promptLength = 1024
+        var tokens = [Int]()
+        for i in 0..<promptLength {
+            tokens.append(100 + (i % 1000))
+        }
+
+        for (pos, tokenId) in tokens.enumerated() {
+            let logits = try model.forward(tokenId: tokenId, position: pos)
+            if pos == 0 || pos == promptLength - 1 || pos % 128 == 0 {
+                let nanCount = logits.filter { $0.isNaN }.count
+                XCTAssertEqual(nanCount, 0, "NaN at pos=\(pos)")
+                print("  pos=\(pos): logits[0..3]=\(logits.prefix(3)) NaN=\(nanCount)")
+            }
+        }
+
+        var genTokens = tokens
+        for i in 0..<5 {
+            let logits = try model.forward(tokenId: genTokens.last ?? 0, position: genTokens.count - 1)
+            let nanCount = logits.filter { $0.isNaN }.count
+            XCTAssertEqual(nanCount, 0, "NaN at gen step \(i)")
+            var maxIdx = 0
+            var maxVal = logits[0]
+            for j in 1..<logits.count {
+                if logits[j] > maxVal { maxVal = logits[j]; maxIdx = j }
+            }
+            genTokens.append(maxIdx)
+            print("  gen[\(i)]: token=\(maxIdx) logit=\(maxVal)")
+        }
+    }
 }