Points the README at statifier-ex

[johnnyt]

• Replaces the active-development notice, which no longer holds: this repo is archived and 1.x sees no further development
• Names riddler/statifier-ex as the rewrite that will publish statifier 2.0 on Hex
• States that 1.9.x stays on Hex and keeps working as-is

Description

Related Issue

Closes #

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Changes Made

Testing

• All existing tests pass
• Added new tests for new functionality
• Coverage remains above 90%
• Tested with SCION test suite (if applicable)
• Tested with W3C test suite (if applicable)

Validation

• mix format - Code is formatted
• mix test --cover - Tests pass with coverage
• mix credo --strict - No credo issues
• mix dialyzer - No dialyzer warnings

SCXML Compliance

• Follows W3C SCXML specification
• Updates feature detection if needed
• Related spec section:

Documentation

• Updated API documentation
• Updated CHANGELOG.md
• Added/updated code examples
• Updated README if needed

Screenshots (if applicable)

Additional Notes