fix(debug): handle mapping edges, stray traps and dying debuggees

Assisted-by: GLM 5.3

debug/tracer.go

@@ -5,6 +5,22 @@
 
 package debug
 
+import (
+	"os"
+	"path/filepath"
+)
+
+// markDead records the target's failure reason in the handshake directory,
+// the death notice the debugger's readiness poll reads.  The poll must not
+// wait on the child while it polls: a wait there could consume the SIGSTOP
+// park the debugger's own waitStopped must receive, hanging the launch, so
+// the file is the only fast death notice that is safe to read.  A debuggee
+// killed without a notice (a crash, SIGKILL) surfaces through the ordinary
+// wait after the poll instead.
+func markDead(tmpDir, reason string) {
+	os.WriteFile(filepath.Join(tmpDir, "dead"), []byte(reason), 0o644)
+}
+
 // tracer abstracts the minimal ptrace operations needed by the breakpoint
 // manager and the stop-information helpers.  The live implementation is
 // *Session (ptrace_linux_amd64.go); tests supply a mock.