absorb: 27 orphaned relibc patches re-applied (Phase 1.0A)

Per local/docs/PATCH-PRESERVATION-AUDIT-2026-07-12.md the relibc
fork was carrying only 34 of 90 patches in local/patches/relibc/.
The other 56 patches' content was silently missing from the fork.

This commit re-applies 27 patches that genuinely still apply
cleanly. Recovery covers:

- eventfd implementation (sys/eventfd.h + eventfd.rs)
- signalfd implementation (sys/signalfd.h + signalfd.rs)
- timerfd implementation (sys/timerfd.h + timerfd.rs)
- bits/eventfd.h header
- spawn() function: cbindgen + stdint fix
- P3-timerfd-cbindgen-fix
- cbindgen language=C fixes for sys/{timerfd,semaphore}
- stdint include chain fixes
- strtold implementation
- dns aaaa getaddrinfo ipv6
- various stack/threading/header threading fixes
- dup3 syscalls
- waitid implementation
- bits/timespec reverse_from
- open_memstream integration

24 files changed.

tests/semaphore/sem_open.c

@@ -0,0 +1,15 @@
+#include <assert.h>
+#include <fcntl.h>
+#include <semaphore.h>
+#include <stdio.h>
+
+int main(void) {
+    sem_t *sem = sem_open("/relibc-test-sem", O_CREAT | O_EXCL, 0600, 1);
+    assert(sem != SEM_FAILED);
+    assert(sem_wait(sem) == 0);
+    assert(sem_post(sem) == 0);
+    assert(sem_close(sem) == 0);
+    assert(sem_unlink("/relibc-test-sem") == 0);
+    puts("sem_open ok");
+    return 0;
+}

tests/sys_shm/shmget.c

@@ -0,0 +1,15 @@
+#include <assert.h>
+#include <stdio.h>
+#include <sys/ipc.h>
+#include <sys/shm.h>
+
+int main(void) {
+    int id = shmget(IPC_PRIVATE, 4096, IPC_CREAT | 0600);
+    assert(id >= 0);
+    void *ptr = shmat(id, 0, 0);
+    assert(ptr != (void *)-1);
+    assert(shmdt(ptr) == 0);
+    assert(shmctl(id, IPC_RMID, 0) == 0);
+    puts("shmget ok");
+    return 0;
+}