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/CMakeLists.txt

@@ -74,7 +74,6 @@ target_link_libraries(veloxd_sched PUBLIC velox::proto velox::core veloxd_store
 # --- veloxd_rpc — the RPC transports + dispatcher ------------------------------------
 add_library(veloxd_rpc STATIC
     src/rpc/runtime_dir.cpp
-    src/rpc/single_instance.cpp
     src/rpc/systemd_activation.cpp
     src/rpc/event_loop.cpp
     src/rpc/event_hub.cpp
@@ -84,6 +83,14 @@ add_library(veloxd_rpc STATIC
     src/rpc/ws_server.cpp
     src/rpc/pairing.cpp
     src/rpc/dispatcher.cpp
+    # Per-OS backends behind daemon/src/rpc/platform/*.hpp (docs/adr/0020,
+    # docs/08-porting.md). Only the Linux backend exists (Phase 0); lane PORT adds
+    # cmake/platform.cmake + platform/macos/ in Phase 1 and this hardcoded list becomes
+    # a velox_platform_sources() call.
+    src/rpc/platform/linux/wakeup.cpp
+    src/rpc/platform/linux/peer_id.cpp
+    src/rpc/platform/linux/instance_lock.cpp
+    src/rpc/platform/linux/runtime_paths.cpp
 )
 add_library(velox::daemon_rpc ALIAS veloxd_rpc)