gate: pass on macOS: tests use /tmp, gatewayd finds its uid without /proc

M5a task 16. The work is Ornith's, from the stopped run; the design
model checked it against the task file and committed it once the
memoryd lint that stopped the gate was fixed (44bdcbd).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.cargo/config.toml

@@ -0,0 +1,5 @@
+# Tests put Unix sockets under `std::env::temp_dir()`. macOS's is long (/var/folders/…/T/), and a
+# socket's path there can pass the 104 bytes macOS allows, so every process Cargo runs, tests
+# included, uses /tmp.
+[env]
+TMPDIR = { value = "/tmp", force = true }