feat: attribute proxy clients by process

[kev1n77]

Summary

• upgrade capture IPC to 0.3 and carry validated loopback client/server endpoints only in the authenticated transport frame
• resolve Proxy client PIDs from exact OS socket matches, ignore sidecar PID claims, and retain request evidence in a bounded session cache for response updates
• keep transport endpoints outside CaptureEnvelope, encrypted persistence, and exports; complete CAP-007 documentation

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• python -m unittest discover -s sidecars/mitmproxy/tests -v
• npm run sidecar:package
• npm run sidecar:verify
• npm run desktop:check
• npm run desktop:build