Add platform compatibility + testing-mode profiles

## Goal
Support configurable behavior profiles so the calculator can mimic standard vs testing constraints.

## Scope
- Add feature-flag profile system:
- Standard profile (full Pretty-Calc scientific feature set)
- Testing profile(s) with selective restrictions
- Initial testing-profile constraints to support:
- disable function definitions
- optionally force degree default
- disable selected advanced functions/categories
- Document browser/platform support matrix and known limitations

## Acceptance Criteria
- Profile selection cleanly gates parser/UI features.
- Restricted features show clear UX messaging (not silent failures).
- Profile behavior is covered by regression tests.
- Documentation includes how to define new profiles.