relibc: full eventfd() implementation + no-stubs policy in AGENTS.md

Replaced P3-sys-eventfd-create.patch (constants-only stub) with:
- P3-eventfd-impl.patch: full eventfd() opening /scheme/event/eventfd
- P3-bits-eventfd.patch: eventfd_t type
- P3-eventfd-cbindgen.patch: generates sys/eventfd.h C header
- P3-bits-eventfd-mod.patch: wires bits_eventfd into header/mod.rs

libwayland: removed eventfd stub from redox.patch — relibc provides it now.
Only meson.build fix + wl_proxy null guards + MSG_NOSIGNAL guard remain.

Documented zero-tolerance stub policy at top of local/AGENTS.md.

local/patches/relibc/P3-bits-eventfd.patch

@@ -0,0 +1,7 @@
+--- /dev/null	2026-05-03 20:55:05.750445686 +0100
++++ b/src/header/bits_eventfd/mod.rs
+@@ -0,0 +1,4 @@
++//! `bits/eventfd.h` — eventfd counter type.
++
++use crate::platform::types::uint64_t;
++pub type eventfd_t = uint64_t;