feat: activate owner-only Unix capture IPC

[kev1n77]

Summary

• activate POSIX Unix-domain capture IPC on macOS and Linux while retaining loopback TCP on Windows
• create the socket inside an eUID-owned 0700 directory, enforce 0600 socket mode, reject symlink/wrong-owner paths, and remove the socket on listener drop
• verify every Unix connection against kernel-reported peer eUID and launched sidecar PID before reading authenticated frames
• add Rust and Tauri-level coverage and update security, threat-model, and progress documentation

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p codeischeap-capture-ipc --all-targets (12 passed)
• cargo clippy -p codeischeap-capture-ipc --all-targets -- -D warnings
• python -m unittest discover -s sidecars/mitmproxy/tests -v (24 passed, 2 Unix-only tests skipped on Windows)

Local environment note

A full Tauri/workspace rebuild is not available on this Windows machine because the installed Git Perl lacks Locale::Maketext::Simple, which is required by the vendored OpenSSL/SQLCipher build. Cross-platform CI is the authoritative native build check.