chore: prepare 0.6.9 release docs and version

[rlippmann]

What changed:

• Updated release-facing documentation for 0.6.9 alignment (REPL precompiler flag usage, reject-first precompiler contract wording, and integration doc consistency).
• Refreshed roadmap wording in docs/DescriptionAndMilestones.md with visible 0.6.x anchor and forward milestones.
• Bumped package version from 0.6.8 to 0.6.9 in pyproject.toml and uv.lock.

Why this change was needed:

• Finalize release readiness and metadata for 0.6.9 while keeping implementation semantics unchanged.