docs: add controller preview example

[rlippmann]

What changed

• Added a new focused example: examples/08_controller_preview_diff.py.
• Added the new example to examples/README.md.
• Added a short controller quick example to README.md showing preview(...), state_diff(...), and step(...).
• Updated examples/integrations/README.md to align the checkpoint continuation version note to context-compiler>=0.7.0.
• Adjusted the README quick example to avoid encouraging direct nested state-shape access and instead show change inspection through state_diff(...).

Why

• Improve discoverability of the 0.7 controller/auditability APIs with one short, user-facing example.
• Keep existing lower-level examples intact while adding a clear path for users who want preview and structural diff behavior.
• Encourage API usage patterns that are less coupled to internal state structure.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)