docs: add example 08 controller preview and diff flow

[rlippmann]

Summary

• Adds examples/08_controller_preview_diff.ts as TS parity for Python examples/08_controller_preview_diff.py.
• Demonstrates auditability flow with:
  • preview(engine, input)
  • state_diff(before, after)
  • explicit check that preview does not mutate live state
  • subsequent real apply with step(engine, input)
• Uses 0.7 helper/constants surface where it improves clarity (is_update, is_clarify, get_decision_state, DECISION_*).
• Updates examples/README.md with Example 08 entry.

Why

Python now includes an auditability/controller example. This keeps TS examples aligned and gives users a clear, minimal dry-run plus apply workflow.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant validation was run (npm test for behavior changes, or docs-only/no-runtime-change)

Notes

Validation details:

• npm test: passed
• If not run, why: n/a