Improve process.isBun DCE test to properly verify code elimination

- Use KEEP/REMOVE/DROP keywords to verify DCE actually removes dead code
- Test now fails on stable Bun (verified), passes with the new feature
- Follows the established DCE test conventions from expectBundled.md

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/options.zig

@@ -1510,6 +1510,14 @@ pub fn definesFromTransformOptions(
                 .value = .{ .e_undefined = .{} },
             }));
         }
+
+        if (!user_defines.contains("process.isBun")) {
+            _ = try environment_defines.getOrPutValue("process.isBun", .init(.{
+                .valueless = false,
+                .original_name = "process.isBun",
+                .value = .{ .e_boolean = .{ .value = true } },
+            }));
+        }
     }
 
     const resolved_defines = try defines.DefineData.fromInput(user_defines, drop, log, allocator);

test/bundler/esbuild/dce.test.ts

@@ -3401,4 +3401,29 @@ describe("bundler", () => {
   //     "/project/node_modules/pkg/styles.css": `button { color: red }`,
   //   },
   // });
+
+  itBundled("dce/ProcessIsBunTargetBun", {
+    files: {
+      "/entry.js": /* js */ `
+        if (process.isBun) {
+          console.log("KEEP_BUN_CODE");
+          function bunOnlyFunction() {
+            return "KEEP_THIS";
+          }
+          console.log(bunOnlyFunction());
+        } else {
+          console.log("REMOVE_NODE_CODE");
+          function nodeOnlyFunction() {
+            return "DROP_THIS";
+          }
+          console.log(nodeOnlyFunction());
+        }
+      `,
+    },
+    target: "bun",
+    dce: true,
+    run: {
+      stdout: "KEEP_BUN_CODE\nKEEP_THIS",
+    },
+  });
 });