fix(remix): make checkbox styles value-comparable

[leoafarias]

Description

CheckboxStyler.onIndeterminate allocated a new identity-based
ContextVariant on every call, causing equivalent fortalCheckboxStyle()
results to compare unequal.

Reuse one shared indeterminate variant while preserving its predicate and
resolved styling behavior. Add regression coverage for equality and matching
hash codes.

Related Issues

Closes #121

Validation

• fvm dart analyze
• fvm dart run melos run ci
  • Generated artifacts reproduced byte-for-byte.
  • Documentation and Fortal parity checks passed.
  • 2,553 Remix, 275 Fortal, and 25 dashboard tests passed.

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Checklist

• My PR includes unit or integration tests for all changed behavior.
• Existing documentation remains accurate; no documentation changes are required.
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.