ci: MemBrowse integration

[michael-membrowse]

Summary

CMRX has build workflows for its integration test targets but no automated way to track how PRs affect memory footprint over time.

This PR adds MemBrowse CI integration for automated memory footprint analysis on pull requests.

Here is an example of the dashboard: https://membrowse.com/public/michael-membrowse/cmrx

This includes:

• membrowse-targets.json - configuration of the targets to be analyzed
• Existing build workflows (stm32g4, stm32h7, rp2040, linux) - extended with pull_request trigger, concurrency settings, and a MemBrowse analysis step at the end of each build
• membrowse-onboard.yml - one-off workflow for building the last N commits to populate historical data on day one
• README badge linking to the MemBrowse dashboard

Currently the integration covers 4 targets: stm32g4, stm32h7, rp2040, and linux.

Setup

1. Create account at https://membrowse.com
2. Create a project and link it to this repo
3. Set the project as publicly available
4. Add MEMBROWSE_API_KEY as a GitHub Actions secret
5. Run the onboard workflow to populate the dashboard with historical data