feat: add Unix capture IPC transport contract

[kev1n77]

Summary

• bump capture IPC and sidecar bundle contract to 0.5
• add bounded absolute Unix socket endpoints to the Rust launcher and Python addon while preserving loopback TCP
• add authenticated Unix listener receive APIs and real frame/ACK tests
• reject old 0.4 bundles before launch and document the staged ACL migration

Verification

• cargo test -p codeischeap-capture-ipc -p codeischeap-sidecar-runtime --all-targets
• cargo clippy -p codeischeap-capture-ipc -p codeischeap-sidecar-runtime --all-targets -- -D warnings
• python -m unittest discover -s sidecars/mitmproxy/tests -v
• WSL Ubuntu test_addon.py, including real AF_UNIX delivery
• python sidecars/mitmproxy/package_sidecar.py
• packaged sidecar probe: startup identity, HTTP/2, cancellation, backpressure, and zero credential canaries

Local environment note

The full SQLCipher workspace rebuild requires Perl modules that are not installed in the local Windows toolchain; the PR CI matrix is the authoritative full workspace and Tauri validation.