feat: bind Linux IPC to sidecar process

[kev1n77]

Summary

• resolve Linux loopback TCP clients through exact /proc socket tuple, inode, and unique PID ownership
• enforce sidecar peer PID verification on Linux and extend the independent-process integration test
• update security, threat model, and progress documentation without closing SEC-003

Verification

• cargo fmt --all -- --check
• cargo test -p codeischeap-process-attribution --all-targets
• cargo clippy -p codeischeap-process-attribution --all-targets -- -D warnings
• cargo check -p codeischeap-process-attribution --all-targets --target x86_64-unknown-linux-gnu
• cargo clippy -p codeischeap-process-attribution --all-targets --target x86_64-unknown-linux-gnu -- -D warnings

Full local workspace/Tauri runs were blocked before compilation by the host's incomplete Perl runtime required by vendored OpenSSL; the PR matrix provides the authoritative full cross-platform run.