README.MD restoration

[LK-Simon]

Problem

The current README.md may have lost useful historical explanations and concrete usage examples that help humans evaluate suitability and ease of use.

Proposed solution

Audit the README.md history, restore all still-applicable removed material, and update every restored statement/example to match current main APIs and dependencies.

This issue tracks restoration only. Every related commit, PR, and discussion message must reference this issue.

[LK-Simon]

README history audit found that the extensive Observer-pattern and Thermometer walkthrough from the 2024 README had been removed from the current document. Restoration commit 63ebabb restores the still-applicable conceptual and usage material, updated for Observable 3.x ownership semantics (ObserverHandlePtr and shared ownership during notification). Obsolete raw-pointer registration examples were intentionally not restored. Tracks #5.