feat: add explicit macOS Wi-Fi bridge source

[AIflow-Labs]

Summary

This is a small follow-up to #191.

It adds an explicit local bridge source for macOS when someone wants a helper-driven connected-AP stream, while keeping the native CoreWLAN scan path as the primary product path.

What changed

• add scripts/macos_wifi_bridge.py
• add --source macos-bridge to the sensing server
• accept only loopback NDJSON records with bridge_kind: connected_rssi
• reject ESP32 binary payloads and disconnected bridge records
• document the bridge as explicit-only and experimental

Notes

• auto never selects macos-bridge
• this does not masquerade as ESP32 or CSI; the API source is labeled wifi-bridge:macos
• this PR is stacked on feat: add native macOS CoreWLAN Wi-Fi source #191 and is easiest to review after that lands

Verification

• cargo test --workspace
• cargo test --workspace --no-default-features
• python3 -m py_compile scripts/macos_wifi_bridge.py
• ./scripts/build-mac-wifi.sh
• rust-port/wifi-densepose-rs/target/tools/macos-wifi-scan/macos-wifi-scan --probe
• manual bridge smoke test against --source macos-bridge
• API smoke check returned source: "wifi-bridge:macos" with live ticking bridge data