test: name exception-action callable result; document convention

Rename the ThrowingCallable from `when` to `result` in the heap-guard
tests and document the exception-assertion structure in CLAUDE.md: the
callable is the // When, assertThatThrownBy(result) is the // Then.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

CLAUDE.md

@@ -54,6 +54,9 @@ Built `.dylib/.so/.dll` are git-ignored; they are regenerated from the submodule
 - JUnit 5 + Mockito (BDDMockito) + AssertJ. Class under test named `sut`. Every test has
   `// Given` / `// When` / `// Then`. BDDMockito only: `given(mock.m()).willReturn(v)` /
   `then(...)` (static-import only `given`/`then`, never `willReturn`/`willThrow`).
+  For exception assertions, capture the action under `// When` as a
+  `ThrowingCallable result = () -> sut.m(...);` and assert it under `// Then` with
+  `assertThatThrownBy(result)` — the callable is the When, the assertion is the Then.
 - Prefer `@ParameterizedTest` over copy-paste (`@ValueSource`, else `@ArgumentsSource`/named cases).
   For large input spaces use seeded-random `@MethodSource` generators — they find corners examples
   miss. Put generators in `RandomArrays` (integration) or a similar util; keep counts low (10–30)

zstd/src/test/java/io/github/dfa1/zstd/ZstdSegmentTest.java

@@ -1,5 +1,6 @@
 package io.github.dfa1.zstd;
 
+import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
@@ -110,8 +111,10 @@ void compressRejectsHeapSource() {
                 MemorySegment dst = arena.allocate(64);
 
                 // When compressing from it
+                ThrowingCallable result = () -> sut.compress(dst, heapSrc);
+
                 // Then it fails fast with a clear message instead of a cryptic FFM error
-                assertThatThrownBy(() -> sut.compress(dst, heapSrc))
+                assertThatThrownBy(result)
                         .isInstanceOf(IllegalArgumentException.class)
                         .hasMessageContaining("native");
             }
@@ -126,8 +129,10 @@ void decompressRejectsHeapDestination() {
                 MemorySegment src = arena.allocate(64);
 
                 // When decompressing into it
+                ThrowingCallable result = () -> sut.decompress(heapDst, src);
+
                 // Then it fails fast with a clear message instead of a cryptic FFM error
-                assertThatThrownBy(() -> sut.decompress(heapDst, src))
+                assertThatThrownBy(result)
                         .isInstanceOf(IllegalArgumentException.class)
                         .hasMessageContaining("native");
             }
@@ -139,8 +144,10 @@ void decompressedSizeRejectsHeapFrame() {
             MemorySegment heapFrame = MemorySegment.ofArray(new byte[8]);
 
             // When reading its decompressed size
+            ThrowingCallable result = () -> Zstd.decompressedSize(heapFrame);
+
             // Then it fails fast with a clear message instead of a cryptic FFM error
-            assertThatThrownBy(() -> Zstd.decompressedSize(heapFrame))
+            assertThatThrownBy(result)
                     .isInstanceOf(IllegalArgumentException.class)
                     .hasMessageContaining("native");
         }