Add CHANGELOG.md (backfilled from README) and CONTRIBUTING.md

[OldCrow]

Summary

The repository has no CHANGELOG.md and no CONTRIBUTING.md. Both are standard artifacts for an open-source v1.0 release.

Action needed

CHANGELOG.md

1. Backfill release notes from the existing version history in README.md (the "Status" section and per-version summaries).
2. Use Keep a Changelog format: ## [vX.Y.Z] — YYYY-MM-DD headers with Added, Fixed, Changed, Removed subsections.
3. Add a [Unreleased] section at the top for ongoing changes.

CONTRIBUTING.md

1. Document the branch and PR workflow.
2. Describe the build prerequisites for each platform (can reference AGENTS.md).
3. Describe how to run tests and the sanitizer build.
4. Note the code style conventions (C++20, #pragma once, the unit type system).
5. Describe how to add a new calculator page (the three-layer pattern).

Files

• CHANGELOG.md — new file at repo root
• CONTRIBUTING.md — new file at repo root
• README.md — link to both from the README