daemon: Phase 0 platform seams (ADR 0020) — eventfd/SO_PEERCRED/instance-lock/XDG behind platform/, zero behaviour change

Introduces the four rpc-layer seams docs/08-porting.md calls for this lane:
platform::Wakeup (eventfd), platform::peer_of (SO_PEERCRED/struct ucred, same-UID
check preserved unchanged), platform::acquire_instance_lock (abstract-namespace
socket), platform::runtime_base_dir/data_base_dir (XDG lookups). Today's Linux
code moves unchanged into daemon/src/rpc/platform/linux/; the seam headers
carry no OS types and no #ifdef.

No fifth interface for timerfd: EventLoop gains a portable add_timer() that
folds the next deadline into poll()'s own timeout, replacing both timerfd
instances in main.cpp — the loop already computes a deadline, so this needs no
per-OS backend at all.

Full suite 59/59 green; no #ifdef outside platform/linux/, no behaviour change.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

daemon/src/rpc/platform/peer_id.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+// Identifies the process on the other end of a connected Unix-domain socket, for the
+// same-UID check that is the Unix transport's authorization boundary (docs/01 §2,
+// CLAUDE.md §4). `SO_PEERCRED`/`struct ucred` is Linux-only; macOS has `getpeereid`,
+// Windows named pipes carry a token instead (docs/08-porting.md "The seams" /
+// "API mapping"). The same-UID check itself is the security property and must not change
+// per-OS (docs/adr/0020 decision 2).
+
+#include <system_error>
+
+namespace velox::daemon::rpc::platform {
+
+struct PeerId {
+    unsigned int uid = 0;
+};
+
+// On success, fills `out` with the peer's identity of the already-connected `fd`. On
+// failure, `out` is untouched and the error_code explains why (matches errno on Linux).
+std::error_code peer_of(int fd, PeerId& out);
+
+}  // namespace velox::daemon::rpc::platform