Dependency refresh: Serializable 0.10.1 / Units 0.2.2 (#2)

[LK-Simon]

Summary

Prepares ESPressio Units 0.2.2 as the first downstream dependency-maintenance release following ESPressio Serializable 0.10.1.

Closes #2.

Changes

• Updates ESPressio Units package metadata to 0.2.2.
• Raises the current opt-in ESPressio Serializable baseline to >=0.10.1 <1.0.0.
• Updates current README dependency/install guidance.
• Updates CHANGELOG.md.
• Refreshes the ESPressio dependency chart.
• Preserves the dependency-free ordinary Units API.
• Adds release-candidate CI covering:
  • existing host Unit/conversion tests;
  • ESP32 compilation of Serializable Unit types against ESPressio Serializable 0.10.1.

Dependency direction

The dependency remains strictly downstream:

ESPressio Serializable
        ^
        |
        | opt-in
        |
ESPressio Units