Make release notes stage-aware

[tdecroyere]

Updates release-note generation so notes match the maturity stage of a release.

• Consecutive releases in the same prerelease stage (dev, alpha, ptr, or rc) include only changes since the previous release.
• The first release after entering a new prerelease stage includes the full current product-version cycle since the previous stable release, or since the beginning of the project when no stable exists.
• Stable releases are cumulative since the previous stable release.
• Updates the README to document the stage-aware behavior.

For the current cycle, 1.0.0-alpha.4 remains incremental from 1.0.0-alpha.3. When the project later moves from alpha to PTR, the first PTR release will contain the full 1.0 release-cycle history for testers joining at that stage.