feat: add demo cli passthrough and docs polish

[rlippmann]

What changed

• Added Demo 5 CLI support for --turns stress testing with deterministic, prefix-extended distractor sequences and unchanged evaluation semantics.
• Added shared --llm-delay handling for direct Demo 5 execution.
• Updated demos.run_demo to forward demo-specific args after -- for single-demo runs.
• Added and updated mocked tests for Demo 5 invariants and runner/direct CLI plumbing.
• Polished demos/README.md flow and wording, including Demo 5 stress-ladder guidance and table readability improvements.
• Applied a small formatting improvement in root README.md by centering the kind column in the decision table.

Why

• Make Demo 5 better for prompt-drift stress testing at larger turn counts while preserving default behavior and success criteria.
• Ensure shared LLM pacing options and demo-specific options can be used cleanly in both direct and runner invocation paths.
• Improve documentation scanability so setup, run instructions, and advanced notes are easier to follow.