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/runtime_paths.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+// Where the OS wants ephemeral runtime state and persistent user data to live, before
+// velox appends its own "/velox" subdirectory and applies the shared 0700-and-owned check
+// (rpc/runtime_dir.cpp — that enforcement is portable POSIX logic and stays there; only
+// "which base directory" is per-OS). Linux: XDG. macOS: $TMPDIR (runtime) and
+// ~/Library/Application Support (data) — see docs/08-porting.md "API mapping".
+
+#include <string>
+#include <system_error>
+
+namespace velox::daemon::rpc::platform {
+
+// The base directory ephemeral runtime state (sockets, lock files) should live under,
+// e.g. "$XDG_RUNTIME_DIR" or "/run/user/<uid>" on Linux. No trailing slash.
+std::error_code runtime_base_dir(std::string& out);
+
+// The base directory persistent user data should live under, e.g. "$XDG_DATA_HOME" or
+// "~/.local/share" on Linux. No trailing slash.
+std::error_code data_base_dir(std::string& out);
+
+}  // namespace velox::daemon::rpc::platform