fix: gateway privilege check before loopback alias commands (PILOT-91)

[matthew-pilot]

What

startProxy() now verifies process privileges before attempting to install loopback aliases via ip/ifconfig. Previously the gateway logged an error but continued as if setup succeeded — subsequent connects failed in mysterious ways because no alias was actually installed.

Why

When the gateway runs as a non-root user, ip addr add / ifconfig lo0 alias fail with permission errors. The gateway silently ignored these failures, leading to hard-to-diagnose connection issues downstream.

Changes

• gateway.go: Add loopbackPrivilegeCheck (euid gate, overridable in tests). startProxy() bails out with a clear error if either the privilege check or the actual alias command fails.
• loopback_linux.go / loopback_darwin.go / loopback_other.go: addLoopbackAlias() now returns error instead of silently logging — the caller decides how to surface the failure.
• zz_coverage_test.go: Tests bypass the privilege gate where they use exec stubs; verify error returns from addLoopbackAlias.

Verification

go build ./...   # clean
go vet ./...     # clean
go test ./...    # PASS (0.054s)

Closes PILOT-91

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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