docs: refine 0.6 compatibility and checkpoint wording

[rlippmann]

Summary

Refines TS 0.6 compatibility wording and release documentation for accuracy against the finalized 0.6 parity surface.

Updates include:

• explicit reference to the Python 0.6.19 fixture/contracts baseline
• clarified semantic compatibility wording
• clarified fixture/contracts validation wording
• clarified continuation-safe checkpoint persistence wording

Why

TS 0.6 now validates against a substantially larger parity surface than earlier README wording described.

These updates make the documented compatibility scope precise and aligned with the actual validated fixture/contracts baseline.

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 tests were run

Notes

No code changes.
No fixture changes.
No CI changes.

Validation:

• npm test