docs: polish 0.7 parity API and fixture wording

[rlippmann]

Summary

• document newly exposed 0.7 parity APIs in README (step, preview, state_diff, DECISION_*, pending clarification visibility)
• clarify checkpoint JSON wrapper wording and fixture snapshot alignment language
• update examples README baseline wording from Python 0.6 to Python 0.7

Why

• keep docs aligned with the implemented 0.7 parity surface
• make parity and fixture snapshot expectations explicit for users and reviewers

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