Causal Chat for Haiku v0.4.0 RC6

RC6 adds an independently runnable, privacy-minimized Haiku installation receipt.

• The packaged checker verifies HPKG checksum and metadata, clean-extracts the native programs, opens the exact BApplication, publishes a unique non-sensitive marker, observes one deduplicated record after the UI/history boundary, and quits cleanly.
• Pasteable receipt stdout contains hashes and coarse OS facts, but no hostname, username, address, local path, credential, or message body.
• The native nats-chat publisher now waits for a server PONG, making publish success a measured server flush rather than a completed local send.
• Existing-window, tiny-/tmp, clean-extraction, privacy, and native x86_64 Haiku paths were exercised.
• Source checks passed: https://github.com/plurigrid/causal/actions/runs/29895103910

This receipt deliberately uses nonlocal.info:4222 only for a generated non-sensitive public marker. That service remains plaintext and unauthenticated; RC6 does not turn it into the separately measured TLS/auth/JetStream service. The durable trusted endpoint is still tailnet-only pending public ingress.