fix(interpreter): fire EXIT trap inside command substitution subshell (#854)

EXIT traps set inside $() now fire and their output is captured by the substitution, matching real bash behavior. Trap state is snapshot/restored so traps don't leak to the parent shell.

Closes #806

Author: chaliy

crates/bashkit/src/interpreter/mod.rs

@@ -5795,12 +5795,31 @@ impl Interpreter {
                             "maximum nesting depth exceeded in command substitution".to_string(),
                         ));
                     }
+                    // Command substitution runs in a subshell: snapshot trap state
+                    // so EXIT traps set inside $() fire and don't leak to parent.
+                    let saved_traps = self.traps.clone();
                     let mut stdout = String::new();
                     for cmd in commands {
                         let cmd_result = self.execute_command(cmd).await?;
                         stdout.push_str(&cmd_result.stdout);
                         self.last_exit_code = cmd_result.exit_code;
                     }
+                    // Fire EXIT trap set inside the command substitution
+                    if let Some(trap_cmd) = self.traps.get("EXIT").cloned()
+                        && saved_traps.get("EXIT") != Some(&trap_cmd)
+                        && let Ok(trap_script) = Parser::with_limits(
+                            &trap_cmd,
+                            self.limits.max_ast_depth,
+                            self.limits.max_parser_operations,
+                        )
+                        .parse()
+                        && let Ok(trap_result) =
+                            self.execute_command_sequence(&trap_script.commands).await
+                    {
+                        stdout.push_str(&trap_result.stdout);
+                    }
+                    // Restore parent trap state
+                    self.traps = saved_traps;
                     self.counters.pop_function();
                     self.subst_generation += 1;
                     let trimmed = stdout.trim_end_matches('\n');

crates/bashkit/tests/spec_cases/bash/command-subst.test.sh

@@ -185,3 +185,41 @@ echo "$result"
 ### expect
 (x)(y)(z)
 ### end
+
+### subst_exit_trap_captured
+# EXIT trap output should be captured inside $(), not leak to parent
+result="$(trap 'echo TRAPPED' EXIT; echo hello)"
+echo "captured=[${result}]"
+### expect
+captured=[hello
+TRAPPED]
+### end
+
+### subst_exit_trap_with_explicit_exit
+# EXIT trap fires on explicit exit inside $()
+result="$(trap 'echo CLEANUP' EXIT; echo data; exit 0)"
+echo "captured=[${result}]"
+### expect
+captured=[data
+CLEANUP]
+### end
+
+### subst_exit_trap_no_leak
+# Trap output must not leak to parent stdout
+out="$(trap 'echo INSIDE' EXIT; echo body)"
+echo "out=[${out}]"
+### expect
+out=[body
+INSIDE]
+### end
+
+### subst_exit_trap_isolation
+# EXIT trap in $() should not affect parent traps
+trap 'echo PARENT' EXIT
+result="$(trap 'echo CHILD' EXIT; echo inner)"
+echo "result=[${result}]"
+trap - EXIT
+### expect
+result=[inner
+CHILD]
+### end