Prepare v4.3.0 release

[PurpleSentinel]

Summary

• set firmware and system-default version metadata to 4.3.0
• promote the post-v4.2 work into the v4.3 changelog
• align README and User Guide release headings
• add a regression test proving params.json, built-in defaults, documentation, the first splash and the hardware-information screen all report the same release

Included since v4.2.0

• live maximum-G display during track sessions
• interactive, page-by-page post-session review with directional G peaks
• Ready-screen motion-based Auto-Dim with configurable dim percentage
• exact hardware Bill of Materials for the cased Waveshare reference variant

Impact

• no user settings are reset or overwritten
• both startup screens now report 4.3.0
• release packaging and on-device identity stay aligned

Validation

• python3 -m unittest discover -s tests -v — 186 tests pass
• git diff --check and Python compilation pass
• connected Waveshare hardware loaded build version 4.3.0
• physical first splash received 4.3.0
• physical hardware splash rendered Firmware v4.3.0
• clean full-application boot completed with the existing saved user settings