Safer JSON IPC path (replace predictable /tmp/spikenaut_signals.ipc)

[rmems]

Problem

JSON adapter defaults to ipc:///tmp/spikenaut_signals.ipc, a world-predictable path on multi-user hosts.

Threat model

Loopback research is fine on single-user machines. On shared hosts, another user can bind/hijack the socket.

Acceptance criteria

• Default under $XDG_RUNTIME_DIR/limen-capital/signals.ipc or /tmp/limen-capital-$UID/signals.ipc with restrictive perms where possible
• Env override LIMEN_JSON_IPC for Julia broadcaster and Rust JSON mode
• Docs updated (wire-protocol-v1.md, wire/README.md)

Related files

• strategy/signal_broadcaster.jl
• execution/src/main.rs