Publish a CHANGELOG and release v1.0

[tschm]

Summary

Add a CHANGELOG.md following Keep a Changelog conventions, tag all past releases retroactively

Motivation

This signals API stability to potential adopters and removes the single biggest production-adoption barrier. Semver guarantees (no breaking changes in minor releases) should accompany this.

Acceptance criteria

• CHANGELOG.md exists at repo root following Keep a Changelog format
• All past releases are tagged retroactively in git
• README documents semver policy