docs: restore CHANGELOG 0.2.0 section and fix release hygiene

[joshua-temple]

What this change does

Release and docs hygiene ahead of the state 1.0 cut (non-breaking):

• Restores the deleted [0.2.0] CHANGELOG section in state/CHANGELOG.md, recovered verbatim from tag state/v0.2.0 (the file had jumped [1.0.0] straight to [0.1.0], leaving a history gap). Historical content is left exactly as it was at that release.
• Reword the [1.0.0] framing so it no longer claims "contracts frozen / additive-only" while its own Changed list contains breaking renames: it now reads as the 0.2.0 to 1.0.0 step finalizing the breaks, additive-only from 1.0.0 onward.
• Fixes the stale Invoke signature in state/README.md (the godoc landing page) and prose in authoring/services.md, both of which still taught the old positional Invoke(src, onDone, onError) form instead of the options form.

(No transport/source CHANGELOGs were added: those modules have no released tags yet, so there was nothing to document.)

Checklist

• Commits signed off (DCO) and conventional
• Docs-only; [0.2.0] restored byte-for-byte; link refs intact